Fix a build failure that occurs with newer versions of Meson when
gdbus-codegen is unavailable.

Taken from upstream:
https://gitlab.com/qemu-project/qemu/-/commit/5ecfb76ccc056eb6127e44268e475827ae73b9e0

diff --git a/configure b/configure
--- a/configure
+++ b/configure
@@ -3341,7 +3341,7 @@ if ! test "$gio" = "no"; then
         gio_cflags=$($pkg_config --cflags gio-2.0)
         gio_libs=$($pkg_config --libs gio-2.0)
         gdbus_codegen=$($pkg_config --variable=gdbus_codegen gio-2.0)
-        if [ ! -x "$gdbus_codegen" ]; then
+        if ! has "$gdbus_codegen"; then
             gdbus_codegen=
         fi
         # Check that the libraries actually work -- Ubuntu 18.04 ships
@@ -5704,6 +5704,8 @@ if test "$gio" = "yes" ; then
     echo "CONFIG_GIO=y" >> $config_host_mak
     echo "GIO_CFLAGS=$gio_cflags" >> $config_host_mak
     echo "GIO_LIBS=$gio_libs" >> $config_host_mak
+fi
+if test "$gdbus_codegen" != "" ; then
     echo "GDBUS_CODEGEN=$gdbus_codegen" >> $config_host_mak
 fi
 echo "CONFIG_TLS_PRIORITY=\"$tls_priority\"" >> $config_host_mak