Clang standard library selection.

* Add new ENV function for selecting stdlib for Clang.
  - The selection is no-op for non-system-clang compilers.
  - Both superenv and stdenv are handled.
* Add new HOMEBREW_CCCFG flag and ccwrapper handling.

Library/ENV/4.3/cc

@@ -161,6 +161,7 @@ class Cmd
     if mode == :cxx
       args << '-std=c++11' if cccfg? 'x'
       args << '-stdlib=libc++' if cccfg? 'g'
+      args << '-stdlib=libstdc++' if cccfg? 'h'
     end
 
     return args unless cccfg? 'O'
@@ -216,6 +217,7 @@ class Cmd
     when :cxxld
       args << '-Wl,-headerpad_max_install_names'
       args << '-stdlib=libc++' if cccfg? 'g'
+      args << '-stdlib=libstdc++' if cccfg? 'h'
     end
     args
   end

Library/Homebrew/extend/ENV/std.rb

@@ -333,6 +333,18 @@ module Stdenv
     end
   end
 
+  def libcxx
+    if compiler == :clang
+      append 'CXX', '-stdlib=libc++'
+    end
+  end
+
+  def libstdcxx
+    if compiler == :clang
+      append 'CXX', '-stdlib=libstdc++'
+    end
+  end
+
   def replace_in_cflags before, after
     CC_FLAG_VARS.each do |key|
       self[key] = self[key].sub(before, after) if has_key?(key)

Library/Homebrew/extend/ENV/super.rb

@@ -99,6 +99,7 @@ module Superenv
     #     make/bsdmake wrappers currently.
     # x - Enable C++11 mode.
     # g - Enable "-stdlib=libc++" for clang.
+    # h - Enable "-stdlib=libstdc++" for clang.
     #
     # On 10.8 and newer, these flags will also be present:
     # s - apply fix for sed's Unicode support
@@ -123,6 +124,18 @@ module Superenv
     end
   end
 
+  def libcxx
+    if self['HOMEBREW_CC'] == 'clang'
+      append 'HOMEBREW_CCCFG', "g", ''
+    end
+  end
+
+  def libstdcxx
+    if self['HOMEBREW_CC'] == 'clang'
+      append 'HOMEBREW_CCCFG', "h", ''
+    end
+  end
+
   # m32 on superenv does not add any CC flags. It prevents "-m32" from being erased.
   def m32
     append 'HOMEBREW_CCCFG', "3", ''