Move old ENV.use_foo? compiler methods to compat

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

Library/Homebrew/compat/compatibility.rb

@@ -80,3 +80,17 @@ end
 
 class UnidentifiedFormula < Formula
 end
+
+module HomebrewEnvExtension extend self
+  def use_clang?
+    compiler == :clang
+  end
+
+  def use_gcc?
+    compiler == :gcc
+  end
+
+  def use_llvm?
+    compiler == :llvm
+  end
+end

Library/Homebrew/extend/ENV.rb

@@ -320,18 +320,6 @@ Please take one of the following actions:
     end
   end
 
-  # don't use in new code
-  # don't remove though, but do add to compatibility.rb
-  def use_clang?
-    compiler == :clang
-  end
-  def use_gcc?
-    compiler == :gcc
-  end
-  def use_llvm?
-    compiler == :llvm
-  end
-
   def make_jobs
     # '-j' requires a positive integral argument
     if self['HOMEBREW_MAKE_JOBS'].to_i > 0