smpeg: enable gtk+ test and point to sdl

This enables non-/usr/local installs.
Fixes Homebrew/homebrew#15517.

Formula/smpeg.rb

@@ -4,19 +4,21 @@ class Smpeg < Formula
   homepage 'http://icculus.org/smpeg/'
   head 'svn://svn.icculus.org/smpeg/trunk'
 
-  depends_on :automake
-  depends_on :libtool
+  depends_on :automake => :build
+  depends_on :libtool => :build
 
   depends_on 'pkg-config' => :build
   depends_on 'sdl'
   depends_on 'gtk+'
 
   def install
+    sdl = Formula.factory("sdl")
     system "./autogen.sh"
     system "./configure", "--disable-debug", "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
-                          "--disable-gtktest",
-                          "--disable-sdltest"
+                          "--disable-sdltest",
+                          # For non-/usr/local installs
+                          "--with-sdl-prefix=#{sdl.opt_prefix}"
     system "make"
     # Install script is not +x by default for some reason
     system "chmod +x ./install-sh"