Remove mingw-specific build flags.

IN_LIBXML is not necessary when configured with --enable-static, at least for libxml-2.8.

Hardcoded libraries to link to are obsolete since knu switched to xml2-config.
Cleanup build tasks for cross compiled binary Windows gems.

Remove cross ruby version checks, since they are obsoleted with the
verify_dll method.

Add verify_dll to the standard rake-compiler 'cross native gem' tasks,
so that it's more compatible with the package_win32_fat_binary command of
the rake-compiler-dev-box.

Suggest using the rake-compiler-dev-box if no cross rubies are installed,
because this is probably the most easiest way to build a proper cross
compilation environment.
Read a list of cross-rubies from .cross_rubies and apply DRY.
Make sure necessary gems are installed for each ruby.
In rake-compiler, versions of running ruby and cross ruby must match.
Specify --conservative to avoid installing the latest bundler many times.
Make sure JRuby has (the latest) bundler.
Add a message in verify_dll.
Use File.expand_path.
Use platform instead of host for case in CrossRuby#target.
Refactor the cross ruby code even more.

- CrossRuby#tool returns the name of a binutils command.
- RUBY_CC_VERSION is built from .cross_ruby.
- Properly escape strings using Regexp.quote and String#shelljoin.
Merge branch 'larskanis-cleanup-mingw-build' into refactor-cross_rubies

Conflicts:
	Rakefile
Strip original so files before copy to avoid complex dependencies.
Merge pull request #1072 from knu/refactor-cross_rubies

Refactor cross rubies (Act 2)