FreeBSD port (37): avoiding dependencies towards /usr/include

[Roland McGrath]

> On FreeBSD, configure is called without --with-headers option.

That is the normal state on all platforms.

> Why? Any #include <limits.h>
> includes                     glibc-xxxx/include/limits.h
> which does #include_next     gcc-3.2's private include/limits.h
> which does #include_next     /usr/include/limits.h.

Something is wrong if this happens.  GCC's limits.h does not use #include_next
unless _GCC_NEXT_LIMITS_H is defined.  The only place that defines that is
GCC's syslimits.h, which is not used under _LIBC_LIMITS_H.  Perhaps your
GCC installation is different from mine somehow, but if so I think that
is where the problem lies.

> Here is a fix.

That change is completely wrong.  Without that -isystem flag, GCC's own
files will not be found as they should.  Things like float.h and stdarg.h
must come from GCC.