build: add -Wexpansion-to-defined

This warning is included in -Wall by clang, but not by GCC (which only
enables it for -Wextra).  Include it in the list of warnings we enable
to minimize the differences between the compilers:
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -1583,7 +1583,7 @@ gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"
 gcc_flags="-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers $gcc_flags"
 gcc_flags="-Wno-missing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"
 gcc_flags="-Wendif-labels -Wno-shift-negative-value $gcc_flags"
-gcc_flags="-Wno-initializer-overrides $gcc_flags"
+gcc_flags="-Wno-initializer-overrides -Wexpansion-to-defined $gcc_flags"
 gcc_flags="-Wno-string-plus-int $gcc_flags"
 # Note that we do not add -Werror to gcc_flags here, because that would
 # enable it for all configure tests. If a configure test failed due