meson: add options to enable/disable various features

And fix build without GLib. Fixes #2.
diff --git a/util/meson.build b/util/meson.build
index 6a6220b..0c2e381 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -57,4 +57,8 @@
     link_with: [libharfbuzz],
     install: true,
   )
+else
+  # Disable tests that use this
+  hb_shape = disabler()
+  hb_subset = disabler()
 endif