Prepare for moving more compiler tests to Meson.  If the full set
of compiler flags is needed in a cc.compiles/cc.links test, it will
be handy to have a variable analogous to QEMU_CFLAGS.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>