gst-python: also provide python 3 overrides

Closes #596.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Formula/gst-python.rb

@@ -10,21 +10,49 @@ class GstPython < Formula
     sha256 "e708aa675d5744fc36608249728290d6d97803cd071bf2ac91d245a9d8e8c996" => :mavericks
   end
 
+  option "without-python", "Build without python 2 support"
+
+  depends_on :python3 => :optional
   depends_on "gst-plugins-base"
-  depends_on "pygobject3"
+
+  if build.with? "python"
+    depends_on "pygobject3"
+  end
+  if build.with? "python3"
+    depends_on "pygobject3" => "with-python3"
+  end
 
   link_overwrite "lib/python2.7/site-packages/gi/overrides"
 
   def install
-    # pygi-overrides-dir switch ensures files don't break out of sandbox.
-    system "./configure", "--disable-dependency-tracking",
-                          "--disable-silent-rules",
-                          "--prefix=#{prefix}",
-                          "--with-pygi-overrides-dir=#{lib}/python2.7/site-packages/gi/overrides"
-    system "make", "install"
+    if build.with?("python") && build.with?("python3")
+      # Upstream does not support having both Python2 and Python3 versions
+      # of the plugin installed because apparently you can load only one
+      # per process, so GStreamer does not know which to load.
+      odie "Options --with-python and --with-python3 are mutually exclusive."
+    end
+
+    Language::Python.each_python(build) do |python, version|
+      # pygi-overrides-dir switch ensures files don't break out of sandbox.
+      system "./configure", "--disable-dependency-tracking",
+                            "--disable-silent-rules",
+                            "--prefix=#{prefix}",
+                            "--with-pygi-overrides-dir=#{lib}/python#{version}/site-packages/gi/overrides",
+                            "PYTHON=#{python}"
+      system "make", "install"
+    end
   end
 
   test do
     system "#{Formula["gstreamer"].opt_bin}/gst-inspect-1.0", "python"
+    Language::Python.each_python(build) do |python, _version|
+      # Without gst-python raises "TypeError: object() takes no parameters"
+      system python, "-c", <<-EOS.undent
+        import gi
+        gi.require_version('Gst', '1.0')
+        from gi.repository import Gst
+        print (Gst.Fraction(num=3, denom=5))
+        EOS
+    end
   end
 end