make fails_with_llvm work when cc is llvm-gcc

Closes Homebrew/homebrew#4693.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/compatibility.rb

@@ -14,6 +14,10 @@ def dump_build_env env
   Homebrew.dump_build_env env
 end
 
+def default_cc
+  MacOS.default_cc
+end
+
 def gcc_42_build
   MacOS.gcc_42_build_version
 end

Library/Homebrew/formula.rb

@@ -265,7 +265,10 @@ class Formula
   end
 
   def fails_with_llvm msg="", data=nil
-    return unless (ENV['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm')
+    unless (ENV['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm')
+      ENV.gcc_4_2 if default_cc =~ /llvm/
+      return
+    end
 
     build = data.delete :build rescue nil
     msg = "(No specific reason was given)" if msg.empty?

Library/Homebrew/utils.rb

@@ -1,3 +1,5 @@
+require 'pathname'
+
 class Tty
   class <<self
     def blue; bold 34; end
@@ -208,6 +210,11 @@ def nostdout
 end
 
 module MacOS extend self
+
+  def default_cc
+    Pathname.new("/usr/bin/cc").realpath.basename.to_s
+  end
+
   def gcc_42_build_version
     `/usr/bin/gcc-4.2 -v 2>&1` =~ /build (\d{4,})/
     if $1