Add C++/Objective-C init in toolchains

tasks/toolchains/clang.rake

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

tasks/toolchains/gcc.rake

 MRuby::Toolchain.new(:gcc) do |conf|
-  conf.cc do |cc|
+  [conf.cc, conf.cxx, conf.objc].each do |cc|
     cc.command = ENV['CC'] || 'gcc'
     cc.flags = [ENV['CFLAGS'] || %w(-g -O3 -Wall -Werror-implicit-function-declaration)]
     cc.include_paths = ["#{root}/include"]

tasks/toolchains/vs2012.rake

 MRuby::Toolchain.new(:vs2012) do |conf|
-  conf.cc do |cc|
+  [conf.cc, conf.cxx].each do |cc|
     cc.command = ENV['CC'] || 'cl.exe'
     cc.flags = [ENV['CFLAGS'] || %w(/c /nologo /W3 /D_DEBUG /MDd /Zi /Od /RTC1 /DDISABLE_GEMS /DHAVE_STRING_H /DNO_GETTIMEOFDAY /D_CRT_SECURE_NO_WARNINGS)]
     cc.include_paths = ["#{root}/include"]