include/c: use __has_attribute

include/c: use __has_attribute

The __has macros are more portable (supported by gcc as well as
clang).

The old __GNUC_PREREQ is deprecated and it should be removed in
future. (Well, the __has macros are supported since gcc 5, so we
should be patient as some old stable distros (e.g. RHEL 7) use gcc
4.x).

This patch helps clang to correctly analyze our xalloc.h stuff.

Signed-off-by: Karel Zak <kzak@redhat.com>