support dylib and so generating
Merge pull request #1 from take-cheeze/dylib_so

Support dylib and shared object generating.
use linker.run and build with :all
implement mruby_dll_ext to MRuby::Build and improve messages
ignore dll building in CrossBuild
Merge pull request #2 from take-cheeze/improvement

Improvement.
update def file
Fix build on windows mingw
Remove specified target
I generated it