won't compile when `-fno-common` is used

[andy5995]

Same cause as theimpossibleastronaut/rmw#280 so the fix shouldn't be too difficult.

/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmd'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmdNV'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: multiple definition of `sock'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: multiple definition of `tty'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: multiple definition of `ttyBufR'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: multiple definition of `ttyBufW'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/telopt.h:40: multiple definition of `telOpt'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/telopt.h:40: first defined here
/usr/bin/ld: atcmd.o:/home/andy/src/modemu2k/_build/src/../../src/cmdarg.h:35: multiple definition of `cmdarg'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/cmdarg.h:35: first defined here
/usr/bin/ld: sockbuf.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:13: multiple definition of `sockBufR'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:13: first defined here
/usr/bin/ld: sockbuf.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:32: multiple definition of `sockBufW'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:32: first defined here
/usr/bin/ld: sockbuf.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: multiple definition of `sock'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: first defined here
/usr/bin/ld: verbose.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmd'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: verbose.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmdNV'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: cmdarg.o:/home/andy/src/modemu2k/_build/src/../../src/cmdarg.h:35: multiple definition of `cmdarg'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/cmdarg.h:35: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: multiple definition of `sock'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmd'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmdNV'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/telopt.h:40: multiple definition of `telOpt'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/telopt.h:40: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: multiple definition of `tty'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: multiple definition of `ttyBufR'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: first defined here
/usr/bin/ld: sock.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: multiple definition of `ttyBufW'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:13: multiple definition of `sockBufR'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:13: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:32: multiple definition of `sockBufW'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sockbuf.h:32: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/telopt.h:40: multiple definition of `telOpt'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/telopt.h:40: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: multiple definition of `tty'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: multiple definition of `ttyBufR'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: multiple definition of `ttyBufW'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmd'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: telopt.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmdNV'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: ttybuf.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: multiple definition of `tty'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:10: first defined here
/usr/bin/ld: ttybuf.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: multiple definition of `ttyBufR'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:22: first defined here
/usr/bin/ld: ttybuf.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: multiple definition of `ttyBufW'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/ttybuf.h:45: first defined here
/usr/bin/ld: ttybuf.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: multiple definition of `sock'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/sock.h:52: first defined here
/usr/bin/ld: lex.yy.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmd'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
/usr/bin/ld: lex.yy.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: multiple definition of `atcmdNV'; modemu2k.o:/home/andy/src/modemu2k/_build/src/../../src/atcmd.h:91: first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:485: modemu2k] Error 1
make[2]: Leaving directory '/home/andy/src/modemu2k/_build/src'
make[1]: *** [Makefile:384: all] Error 2
make[1]: Leaving directory '/home/andy/src/modemu2k/_build/src'
make: *** [Makefile:455: all-recursive] Error 1

[andy5995]

Should build on gcc-10 now.