Fix c++ compiler for windows platforms

x86_64-w64-mingw32-cpp,i686-w64-mingw32-cpp does not compile c++ sourcese.

Author: kjunichi

build_config.rb

@@ -67,7 +67,7 @@ def gem_config(conf)
   [conf.cc, conf.linker].each do |cc|
     cc.command = 'x86_64-w64-mingw32-gcc'
   end
-  conf.cxx.command      = 'x86_64-w64-mingw32-cpp'
+  conf.cxx.command      = 'x86_64-w64-mingw32-g++'
   conf.archiver.command = 'x86_64-w64-mingw32-gcc-ar'
   conf.exts.executable  = ".exe"
 
@@ -83,7 +83,7 @@ def gem_config(conf)
   [conf.cc, conf.linker].each do |cc|
     cc.command = 'i686-w64-mingw32-gcc'
   end
-  conf.cxx.command      = 'i686-w64-mingw32-cpp'
+  conf.cxx.command      = 'i686-w64-mingw32-g++'
   conf.archiver.command = 'i686-w64-mingw32-gcc-ar'
   conf.exts.executable  = ".exe"