Add support for Meson as alternative/parallel build system

https://github.com/mesonbuild/meson

With contributions from:

Tim-Philipp Müller <tim@centricular.com>
Jussi Pakkanen <jpakkane@gmail.com> (original port)

Highlights of the features provided are:
* Faster builds on Linux (~40-50% faster)
* The ability to build with MSVC on Windows
* Generate Visual Studio project files
* Generate XCode project files
* Much faster builds on Windows (on-par with Linux)
* Seriously fast configure and building on embedded

... and many more. For more details see:

http://blog.nirbheek.in/2016/05/gstreamer-and-meson-new-hope.html
http://blog.nirbheek.in/2016/07/building-and-developing-gstreamer-using.html

Building with Meson should work on both Linux and Windows, but may
need a few more tweaks on other operating systems.

ext/mpg123/meson.build

@@ -0,0 +1,16 @@
+mpg123_sources = [
+  'gstmpg123audiodec.c',
+]
+
+mpg123_dep = dependency('libmpg123', version : '>= 1.3', required : false)
+
+if mpg123_dep.found()
+  gstmpg123 = library('gstmpg123',
+    mpg123_sources,
+    c_args : ugly_args,
+    include_directories : [configinc],
+    dependencies : [gstaudio_dep, mpg123_dep],
+    install : true,
+    install_dir : plugins_install_dir,
+  )
+endif