be #included at all.
* aclocal.m4 (GUILE_HEADER_LIBC_WITH_UNISTD): New autoconf macro.
* configure.in: Call it.
* acconfig.h (LIBC_H_WITH_UNISTD_H): New CPP symbol.
* posix.c, filesys.c: Use its results to decide whether or not to
#include <libc.h>.
* configure, scmconfig.h.in: Rebuilt with autoconf and
autoheader.
#define _POSIX_SOURCE before #including it.
* aclocal.m4 (GUILE_STRUCT_UTIMBUF): New test.
* acconfig.h: New comment text for above CPP symbol.
* configure.in: Call it.
* posix.c: #define _POSIX_SOURCE if it seems necessary.