_POSIX_C_SOURCE causes trouble on FreeBSD

[Andreas Vögele]

If the macro _POSIX_C_SOURCE, which is used in stime.c and filesys.c, 
is defined FreeBSD doesn't declare XOPEN and BSD functions like 
strptime and fchmod. Since _POSIX_C_SOURCE also causes trouble when 
building Guile with Mingw I suggest to remove this definition 
completely or to use

#ifdef __hpux
#define _POSIX_C_SOURCE 199506L
#endif

I'd remove the definition. AFAIK it's only required on HP-UX and it 
seems that nobody besides me has ever tried to use Guile on HP-UX. I 
can always add this definition to CPPFLAGS before running configure. 
Furthermore, I think it's more important to get Guile running on 
FreeBSD and to avoid an ifdef mess in the source code.