Remove GCC < 4.8 checks
Hi,
Since commit efc6c07 ("configure: Add a test for the minimum compiler version"),
QEMU explicitely depends on GCC >= 4.8.
v2:
- include reviewed Philippe earlier series
- drop problematic patch to replace GCC_FMT_ATTR, but tweak the check to be clang
- replace QEMU_GNUC_PREREQ with G_GNUC_CHECK_VERSION
- split changes
- add patches to drop __GNUC__ checks (clang advertizes itself as 4.2.1, unless
-fgnuc-version=0)