Adds option to build opencv with the qt backend

See issue 9444

Closes Homebrew/homebrew#9445.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/opencv.rb

@@ -33,13 +33,19 @@ class Opencv < Formula
     DATA
   end
 
+  depends_on 'qt' if ARGV.include? '--with-qt'
+
   def options
-    [["--32-bit", "Build 32-bit only."]]
+    [
+      ["--32-bit", "Build 32-bit only."],
+      ["--with-qt", "Build qt backend."]
+    ]
   end
 
   def install
     args = std_cmake_parameters.split
     args << "-DOPENCV_EXTRA_C_FLAGS='-arch i386 -m32'" if ARGV.build_32_bit?
+    args << "-DWITH_QT=ON" if ARGV.include? "--with-qt"
 
     # The CMake `FindPythonLibs` Module is dumber than a bag of hammers when
     # more than one python installation is available---for example, it clings