load pre-compile configs from config files

When create a pre-compiled gem in MinGW MSYS shell passing Ruby or OpenCV's paths,
the paths are converted to be invalid by MSYS shell.

E.g.
  When we run the following command
    $ rake gem:precompile RUBIES=C:/ruby19/bin/ruby.exe,C:/ruby20/bin/ruby.exe
  The following paths are expected
    ENV['RUBIES'] = "C:/ruby19/bin/ruby.exe,C:/ruby20/bin/ruby.exe"
  But we get the following
    ENV['RUBIES'] = "C;C:\\MinGW\\msys\\1.0\\ruby19\\bin\\ruby.exe,C;C:\\MinGW\\msys\\1.0\\ruby20\\bin\\ruby.exe"

This fix is for avoiding the problem.

Rakefile

@@ -47,36 +47,27 @@ task 'gem:precompile' => ['gem'] do
   installer = Gem::Installer.new(gemfile)
   installer.unpack(target_dir)
 
-  rubies = ENV['RUBIES'] ? ENV['RUBIES'].split(',') : [Gem.ruby]
-  args = ENV['EXT_OPTS'] ? ENV['EXT_OPTS'].split(',') : []
-
   gemspec = installer.spec
   extension = gemspec.extensions[0]
   gemspec.extensions.clear
-  gemspec.platform = ENV['PLATFORM'] || Gem::Platform::CURRENT
+
+  config = ENV['CONFIG'] ?  YAML.load_file(ENV['CONFIG']) : {}
+  rubies = config['rubies'] || [Gem.ruby]
+  args = config['extopts'] || []
+  gemspec.platform = config['platform'] || Gem::Platform::CURRENT
 
   multi = rubies.size > 1
   rubies.each { |ruby|
-    results = []
-
-    # Convert MinGW's drive letters to Windows' ones
-    #   e.g. /c/ruby/bin/ruby.exe => c:/ruby/bin/ruby.exe
-    ruby.gsub!(/^\/([a-zA-Z])\//, '\1:/')
-
     lib_dir = 'lib'
     if multi
       major, minor, _ = `#{ruby} -e "print RUBY_VERSION"`.chomp.split('.')
       lib_dir = File.join(lib_dir, [major, minor].join('.'))
     end
 
-    target_platform = `#{ruby} -e "print RUBY_PLATFORM"`
-    # Convert MinGW's drive letters to Windows' ones
-    #   e.g. --with-opencv-dir=/c/path/to/opencv => --with-opencv-dir=c:/path/to/opencv
-    args.map! { |a| a.gsub(/=\/([a-zA-Z])\//, '=\1:/') } if target_platform =~ /mingw/
-
-    make_cmd = (target_platform =~ /mswin/) ? 'nmake' : 'make'
+    make_cmd = (`#{ruby} -e "print RUBY_PLATFORM"` =~ /mswin/) ? 'nmake' : 'make'
     Dir.chdir(target_dir) {
       cmd = [ruby, extension, *args].join(' ')
+      results = []
       Gem::Ext::ExtConfBuilder.run(cmd, results)
       Gem::Ext::ExtConfBuilder.make('', results)
 

config.yml

@@ -0,0 +1,7 @@
+platform: mingw32
+rubies:
+  - C:/ruby-1.9.3-p392-mingw32/bin/ruby.exe
+  - C:/ruby-2.0.0-p0-mingw32/bin/ruby.exe
+extopts:
+  - --with-opencv-include=C:/opencv-2.4.5/build/include
+  - --with-opencv-lib=C:/opencv-2.4.5/build/x86/mingw/lib