Add clang compiler option

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/--env.rb

@@ -9,8 +9,9 @@ module Homebrew extend self
   end
 
   def dump_build_env env
+    puts %["--use-clang" was specified] if ARGV.include? '--use-clang'
     puts %["--use-llvm" was specified] if ARGV.include? '--use-llvm'
-    puts "\"--use-gcc\" was specified" if ARGV.include? '--use-gcc'
+    puts %["--use-gcc" was specified] if ARGV.include? '--use-gcc'
 
     %w[ CC CXX LD ].each do |k|
       value = env[k]

Library/Homebrew/extend/ENV.rb

@@ -18,7 +18,11 @@ module HomebrewEnvExtension
       self['CMAKE_PREFIX_PATH'] = "#{HOMEBREW_PREFIX}"
     end
 
-    if MACOS_VERSION >= 10.6 and (self['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm')
+    if MACOS_VERSION >= 10.6 and ARGV.include? '--use-clang'
+      self['CC'] = "#{MacOS.xcode_prefix}/usr/bin/clang"
+      self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/clang++"
+      cflags = ['-O3'] # -O4 makes the linker fail on some formulae
+    elsif MACOS_VERSION >= 10.6 and (self['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm')
       self['CC'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-gcc"
       self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-g++"
       cflags = ['-O4'] # link time optimisation baby!