gtkprintbackendcups.c uses

#ifdef __linux__
#define _GNU_SOURCE
#endif

and then uses strptime(). On systems that use glibc but are not Linux (e.g. Debian GNU/kFreeBSD and GNU/Hurd) this results in:

/build/gtk+3.0-KGFa7z/gtk+3.0-3.12.2/./modules/printbackends/cups/gtkprintbackendcups.c: In function 'localtime_to_utctime':
/build/gtk+3.0-KGFa7z/gtk+3.0-3.12.2/./modules/printbackends/cups/gtkprintbackendcups.c:5569:9: warning: implicit declaration of function 'strptime' [-Wimplicit-function-declaration]
         end = strptime (local_time, formats_0[i], &local_print_time);
         ^
/build/gtk+3.0-KGFa7z/gtk+3.0-3.12.2/./modules/printbackends/cups/gtkprintbackendcups.c:5569:13: warning: assignment makes pointer from integer without a cast
         end = strptime (local_time, formats_0[i], &local_print_time);
             ^
/build/gtk+3.0-KGFa7z/gtk+3.0-3.12.2/./modules/printbackends/cups/gtkprintbackendcups.c:5571:13: warning: assignment makes pointer from integer without a cast
         end = strptime (local_time, formats_1[i], &local_print_time);
             ^

and possibly truncating pointers to 32 bits, causing runtime failure.

I think it would be better to use AC_USE_SYSTEM_EXTENSIONS in configure.ac, like GLib does since Bug #684123 was fixed.