Detect X11 when ENV.x11 is requested; dump in --config.

Library/Homebrew/brew.h.rb

@@ -504,3 +504,7 @@ def llvm_build
     $1.to_i
   end
 end
+
+def x11_installed?
+  Pathname.new('/usr/X11/lib/libpng.dylib').exist?
+end

Library/Homebrew/extend/ENV.rb

@@ -164,6 +164,8 @@ module HomebrewEnvExtension
     append_to_cflags ' -I/usr/include/libxml2'
   end
   def x11
+    opoo "You do not have X11 installed, this formula may not build." if not x11_installed?
+    
     # CPPFLAGS are the C-PreProcessor flags, *not* C++!
     append 'CPPFLAGS', '-I/usr/X11R6/include'
     append 'LDFLAGS', '-L/usr/X11R6/lib'

bin/brew

@@ -65,6 +65,7 @@ Ruby: #{RUBY_VERSION}-#{RUBY_PATCHLEVEL}
 GCC-4.2: #{gcc ? "build #{gcc}" : "N/A"}
 LLVM: #{llvm ? "build #{llvm}" : "N/A" }
 MacPorts or Fink? #{macports_or_fink_installed?}
+X11 installed? #{x11_installed?}
 EOS
 end