ENV.cxx11: Do not use -stdlib=libc++ for Linuxbrew

Closes #63.

Signed-off-by: Shaun Jackman <[email protected]>

Author: Teddy Reed

Library/Homebrew/extend/ENV/std.rb

@@ -320,7 +320,7 @@ def universal_binary
   def cxx11
     if compiler == :clang
       append "CXX", "-std=c++11"
-      append "CXX", "-stdlib=libc++"
+      append "CXX", "-stdlib=libc++" if OS.mac?
     elsif compiler =~ /gcc-(4\.(8|9)|5)/
       append "CXX", "-std=c++11"
     else