Meson already knows to test with the positive form of the flag, which
simplifies the test.  Warnings are now tested explicitly for the C++
compiler, instead of hardcoding those that are only available for
the C language.

At this point all compiler flags in QEMU_CFLAGS are global and only
depend on the OS.  No feature tests are performed in configure.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>