configure: fix detection of gdbus-codegen

"pkg-config --variable=gdbus_codegen gio-2.0" returns "gdbus-codegen",
and it does not pass test -x (which does not walk the path).

Meson 0.58.0 notices that something is iffy, as the dbus_vmstate1
assignment in tests/qtest/meson.build uses an empty string as the
command, and fails very eloquently:

../tests/qtest/meson.build:92:2: ERROR: No program name specified.

Use the "has" function instead of test -x, and fix the generation
of config-host.mak since meson.build expects that GDBUS_CODEGEN
is absent, rather than empty, if the tool is unavailable.

Reported-by: Sebastian Mitterle <smitterl@redhat.com>
Fixes: #178
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 5ecfb76ccc)
Signed-off-by: Michael Roth <michael.roth@amd.com>

configure

@@ -3331,7 +3331,7 @@ 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 "$gdbus_codegen"; then
             gdbus_codegen=
         fi
         # Check that the libraries actually work -- Ubuntu 18.04 ships
@@ -5678,6 +5678,8 @@ if test "$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