ENV: mark gcc-6 as supporting C++11 (#349)

Add SharedEnvExtension#gcc_with_cxx11_support? to centralise the
logic for checking whether a compiler is known to support C++11.

Update logic to accept GCC 4.8 and above (including 6). Thereby also
address oversight in #163 where support for GCC 6 was added without
updating the C++11 compiler whitelist.

Add tests for Superenv#cxx11.

Closes #346.

Library/Homebrew/extend/ENV/shared.rb

@@ -322,4 +322,9 @@ module SharedEnvExtension
       raise "Non-Apple GCC can't build universal binaries"
     end
   end
+
+  def gcc_with_cxx11_support?(cc)
+    version = cc[/^gcc-(\d+(?:\.\d+)?)$/, 1]
+    version && Version.new(version) >= Version.new("4.8")
+  end
 end

Library/Homebrew/extend/ENV/std.rb

@@ -287,7 +287,7 @@ module Stdenv
     if compiler == :clang
       append "CXX", "-std=c++11"
       append "CXX", "-stdlib=libc++"
-    elsif compiler =~ /gcc-(4\.(8|9)|5)/
+    elsif gcc_with_cxx11_support?(compiler)
       append "CXX", "-std=c++11"
     else
       raise "The selected compiler doesn't support C++11: #{compiler}"

Library/Homebrew/extend/ENV/super.rb

@@ -303,11 +303,10 @@ module Superenv
   end
 
   def cxx11
-    case homebrew_cc
-    when "clang"
+    if homebrew_cc == "clang"
       append "HOMEBREW_CCCFG", "x", ""
       append "HOMEBREW_CCCFG", "g", ""
-    when /gcc-(4\.(8|9)|5)/
+    elsif gcc_with_cxx11_support?(homebrew_cc)
       append "HOMEBREW_CCCFG", "x", ""
     else
       raise "The selected compiler doesn't support C++11: #{homebrew_cc}"

Library/Homebrew/test/test_ENV.rb

@@ -141,4 +141,33 @@ class SuperenvTests < Homebrew::TestCase
     assert_equal [], @env.deps
     assert_equal [], @env.keg_only_deps
   end
+
+  def test_unsupported_cxx11
+    %w[gcc gcc-4.7].each do |compiler|
+      @env["HOMEBREW_CC"] = compiler
+      assert_raises do
+        @env.cxx11
+      end
+      refute_match "x", @env["HOMEBREW_CCCFG"]
+    end
+  end
+
+  def test_supported_cxx11_gcc_5
+    @env["HOMEBREW_CC"] = "gcc-5"
+    @env.cxx11
+    assert_match "x", @env["HOMEBREW_CCCFG"]
+  end
+
+  def test_supported_cxx11_gcc_6
+    @env["HOMEBREW_CC"] = "gcc-6"
+    @env.cxx11
+    assert_match "x", @env["HOMEBREW_CCCFG"]
+  end
+
+  def test_supported_cxx11_clang
+    @env["HOMEBREW_CC"] = "clang"
+    @env.cxx11
+    assert_match "x", @env["HOMEBREW_CCCFG"]
+    assert_match "g", @env["HOMEBREW_CCCFG"]
+  end
 end