GNUstep sources are built without optimization and CodeView debug info

[triplef]

tools-make usually uses CFLAGS generated by Autoconf to set the internal OPTFLAG variable to -g -O2. However on Windows Autoconf doesn’t seem to recognize Clang as a GNU-compatible compiler (checking whether the compiler supports GNU C... no), which causes CFLAGS to be just -g and sources to be built without optimizations.

Additionally -gcodeview is required on Windows to generate full debug info. This is fixed in gnustep/tools-make#39.