Missing $(srcdir) usage in install-headers target

[pmandin]

Hello,

When compiling from a different directory, make install fails because current directory is not where source resides.
Simply adding $(srcdir)/ fixes it when copying headers, i.e for each header file:
cp -a $(srcdir)/include/bits/fenv.h
instead of
cp -a include/bits/fenv.h
in Makefile.in