meson: Added meson.build for gsm

https://bugzilla.gnome.org/show_bug.cgi?id=784646

ext/gsm/meson.build

+gsm_dep = cc.find_library('gsm', required : false)
+
+if gsm_dep.found() and (cc.has_header_symbol('gsm.h', 'gsm_create') or
+			cc.has_header_symbol('gsm/smg.h', 'gsm_create'))
+  gstgsm = library('gstgsm',
+    ['gstgsm.c', 'gstgsmenc.c', 'gstgsmdec.c'],
+    c_args : gst_plugins_bad_args,
+    include_directories : [configinc],
+    dependencies : [gstaudio_dep, gsm_dep],
+    install : true,
+    install_dir : plugins_install_dir,
+  )
+endif

ext/meson.build

@@ -16,7 +16,7 @@ subdir('flite')
 subdir('fluidsynth')
 subdir('gl')
 #subdir('gme')
-#subdir('gsm')
+subdir('gsm')
 subdir('gtk')
 subdir('hls')
 subdir('iqa')