openct fails to build against glibc-2.8

[vapier]

some openct code uses GNU-specific features but doesnt define _GNU_SOURCE

fix is to either add _GNU_SOURCE to CPPFLAGS in the configure script, or just add '#define _GNU_SOURCE' to the affected files

for example:

socket.c: In function 'ct_socket_getcreds':
socket.c:298: error: storage size of 'creds' isn't known
socket.c:298: warning: unused variable 'creds'

[alonbl]

Thanks!

Fixed in changeset:1059.

Added AC_GNU_SOURCE.