pango: Add fallback dependency

When we'll depend on Meson >=0.55 this can be replaced by [provide]
section in gst-build's pango.wrap file. The dependency variable name can
also be dropped when pango is updated to use override_dependency:
https://gitlab.gnome.org/GNOME/pango/-/merge_requests/250

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/844>

ext/pango/meson.build

@@ -6,7 +6,9 @@ pango_sources = [
   'gsttimeoverlay.c',
 ]
 
-pango_dep = dependency('pangocairo', version : '>=1.22.0', required : get_option('pango'))
+pango_dep = dependency('pangocairo', version : '>=1.22.0', required : get_option('pango'),
+  fallback: ['pango', 'libpangocairo_dep'],
+  default_options: ['cairo=enabled'])
 
 if pango_dep.found()
   gstpango = library('gstpango',