configure: check for gdbus-codegen presence

Some distros ship gdbus-codegen separately for gio headers/pc...
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20200110112725.689401-1-marcandre.lureau@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

configure

@@ -3702,6 +3702,9 @@ if $pkg_config --atleast-version=$glib_req_ver gio-2.0; then
     gio_cflags=$($pkg_config --cflags gio-2.0)
     gio_libs=$($pkg_config --libs gio-2.0)
     gdbus_codegen=$($pkg_config --variable=gdbus_codegen gio-2.0)
+    if [ ! -x "$gdbus_codegen" ]; then
+        gdbus_codegen=
+    fi
 else
     gio=no
 fi