Fix header breakage with _XOPEN_SOURCE.

Fix header breakage with _XOPEN_SOURCE.

convert-objects.c sets _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED before
including <time.h>, in order to get the declaration of strptime().
This leads to breakage in cache.h, due to S_ISLNK and S_IFLNK no longer
being defined by <sys/stat.h>.  These definitions are protected by the
__USE_BSD symbol, which is not set when _XOPEN_SOURCE is set.  Moving
the #defines and #include <time.h> below all other #includes does not
fix the problem, however, since now _USE_XOPEN, which protects the
declaration of strptime(), is now not defined (don't ask!).

The fix is to #define _GNU_SOURCE, which enables the definition of
practically everything.

Signed-off-by: Ramsay Allan Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>