convert-objects: set _XOPEN_SOURCE to 600
convert-objects: set _XOPEN_SOURCE to 600

Otherwise OpenBSD header files drop S_ISLNK() definition which is used in
an inline defined in cache.h

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git