_GNU_SOURCE is already defined in QEMU_CFLAGS which
is passed to gcc in shell function compile_prog.

Removing the definition from several checks avoids compiler warnings
(which are now written to config.log).
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>