Merge pull request #1739 from take-cheeze/clang_cxx

Specialize C++ compiler in clang toolchain.

tasks/toolchains/clang.rake

 MRuby::Toolchain.new(:clang) do |conf|
   toolchain :gcc
 
-  [conf.cc, conf.cxx, conf.objc, conf.asm].each do |cc|
+  [conf.cc, conf.objc, conf.asm].each do |cc|
     cc.command = ENV['CC'] || 'clang'
   end
+  conf.cxx.command = ENV['CXX'] || 'clang++'
   conf.linker.command = ENV['LD'] || 'clang'
 end