GCC supports "#pragma GCC diagnostic" since version 4.6, and
Clang seems to support it, too, since its early versions 3.x.
That means that our minimum required compiler versions all support
this pragma already and we can remove the test from configure and
all the related #ifdefs in the code.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200710045515.25986-1-thuth@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>