This reverts commit e4e469a0

.

It looks like we get to choose between breaking the clang build or the
gcc build. With gcc, we need to ensure that enter xorg/os.h with the
right set of HAVE_STRNDUP et al guards, e.g.

In file included from /usr/include/string.h:630:0,
                 from /usr/include/xorg/os.h:53,
                 from /usr/include/xorg/misc.h:116,
                 from fd.c:34:
/usr/include/xorg/os.h:590:1: error: expected identifier or '(' before '__extension__'
 strndup(const char *str, size_t n);

Reported-by: Arkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>