sdl: Add optional support for X11 video driver

The Quartz driver in the SDL 1.2 library doesn't work with wrappers
(like go, haskell, ...) which hijack the main() function (as opposed to
using SDL_main). One solution is to use the (deprecated) X11 SDL video
driver backend, in combination with the open source XQuartz server.
Homebrew's current formula builds SDL without the X11 backend by
default. This change adds an optional installation option that
re-enables the X11 backend.

Closes Homebrew/homebrew#22108.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/sdl.rb

@@ -2,17 +2,19 @@ require 'formula'
 
 class Sdl < Formula
   homepage 'http://www.libsdl.org/'
+  head 'http://hg.libsdl.org/SDL', :branch => 'SDL-1.2', :using => :hg
   url 'http://www.libsdl.org/release/SDL-1.2.15.tar.gz'
   sha1 '0c5f193ced810b0d7ce3ab06d808cbb5eef03a2c'
 
-  head 'http://hg.libsdl.org/SDL', :branch => 'SDL-1.2', :using => :hg
+  option 'with-x11-driver', 'Compile with support for X11 video driver'
+  option :universal
 
   if build.head?
     depends_on :automake
     depends_on :libtool
   end
 
-  option :universal
+  depends_on :x11 if build.with? 'x11-driver'
 
   def patches
     # Fix for a bug preventing SDL from building at all on OSX 10.9 Mavericks
@@ -34,7 +36,7 @@ class Sdl < Formula
     args << "--disable-nasm" unless MacOS.version >= :mountain_lion # might work with earlier, might only work with new clang
     # LLVM-based compilers choke on the assembly code packaged with SDL.
     args << '--disable-assembly' if ENV.compiler == :llvm or (ENV.compiler == :clang and MacOS.clang_build_version < 421)
-    args << '--without-x'
+    args << "--without-x" if build.without? 'x11-driver'
 
     system './configure', *args
     system "make install"