meson: build aalib plugin

https://bugzilla.gnome.org/show_bug.cgi?id=784134

ext/aalib/meson.build

@@ -0,0 +1,14 @@
+# Very much not going to implement all kinds of logic around aalib-config
+# or cater for non-standard prefixes.
+if cc.has_header('aalib.h')
+  libaa_dep = cc.find_library('aa', required : false)
+  if libaa_dep.found()
+    library('gstaasink', 'gstaasink.c',
+      c_args : gst_plugins_good_args,
+      link_args : noseh_link_args,
+      include_directories : [configinc],
+      dependencies : [gstvideo_dep, gstbase_dep, libaa_dep],
+      install : true,
+      install_dir : plugins_install_dir)
+  endif
+endif

ext/meson.build

@@ -1,4 +1,4 @@
-# subdir('aalib')
+subdir('aalib')
 subdir('cairo')
 subdir('flac')
 subdir('gdk_pixbuf')