Resolve multi-level gcc symlinks

In later XCodes, gcc is a symlink to gcc-4.2 which is itself a symlink
to llvm-gcc, so it is necessary to resolve multiple symlinks.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/extend/ENV.rb

@@ -90,8 +90,7 @@ module HomebrewEnvExtension
     @compiler = :gcc
 
     raise "GCC could not be found" if args[:force] and not File.exist? ENV['CC'] \
-                                   or (File.symlink? ENV['CC'] \
-                                   and File.readlink(ENV['CC']) =~ /llvm/)
+                                   or (Pathname.new(ENV['CC']).realpath.to_s =~ /llvm/)
   end
   alias_method :gcc_4_2, :gcc