libsmack: don't define __GNU_SOURCE in the code

libsmack: don't define __GNU_SOURCE in the code

__GNU_SOURCE is a glibc internal and should not be used directly.
It was already provided by AC_USE_SYSTEM_EXTENSIONS in configure.ac, so
it can be safely dropped.
Use more explicit AC_GNU_SOURCE in configure.ac instead (on Linux systems
they should be equivalent).

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>