ENV: fix llvm-gcc regexp

Fixes: Homebrew/homebrew#8890.
Closes Homebrew/homebrew#8893.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Library/Homebrew/extend/ENV.rb

@@ -135,7 +135,7 @@ module HomebrewEnvExtension
 
     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')
+                                   and File.readlink(ENV['CC']) =~ /llvm/)
   end
   alias_method :gcc_4_2, :gcc