On some hosts (eg Ubuntu Bionic) pkg-config returns a set of
libraries for gio-2.0 which don't actually work when compiling
statically. (Specifically, the returned library string includes
-lmount, but not -lblkid which -lmount depends upon, so linking
fails due to missing symbols.)

Check that the libraries work, and don't enable gio if they don't,
in the same way we do for gnutls.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200928160402.7961-1-peter.maydell@linaro.org