use CROSS_PREFIX determined by rake-compiler also for g++

Rakefile.cross

@@ -391,6 +391,7 @@ task :mingw32 do
     warn "Please refer to your distribution/package manager documentation about installation."
     fail
   end
+  ENV['CROSS_PREFIX'] = Rake::ExtensionCompiler.mingw_host
 end
 
 

ext/fox16/extconf.rb

@@ -79,7 +79,7 @@ def do_rake_compiler_setup
     have_library( 'opengl32' ) && append_library( $libs, 'opengl32' )
     have_library( 'winspool', 'EnumPrintersA') && append_library( $libs, 'winspool' )
 
-    CONFIG['CC'] += "\nCXX=i586-mingw32msvc-g++" # Hack CXX into Makefile for cross compilation
+    CONFIG['CC'] += "\nCXX=#{ENV['CROSS_PREFIX']}-g++" # Hack CXX into Makefile for cross compilation
     $CFLAGS += " -D_SYS_TIME_H_" # fix incompatible types for gettimeofday()
   elsif RUBY_PLATFORM =~ /mingw/
     $CFLAGS = $CFLAGS + " -I/usr/local/include"