Delete tcl, tk dependency with NOTK=1 on Ruby20, 21

ext/tk will not be built with NOTK=1. It is same as Ruby 18, 19.
Merge pull request #225 from shirosaki/notk

Delete tcl, tk dependency with NOTK=1 on Ruby20, 21
Update README with DevKit req. for Ruby 2.x

Both Ruby 2.0.x and 2.1.x are built using a different version of DevKit than the default one.

Modify README.txt to reflect this.

Closes #231
Use msys2(32bit) instead of old msys

Parallel make hangs due to msys bug.
http://sourceforge.net/p/mingw/bugs/1950/

msys2 is updated version of msys and the bug seems to be fixed.
http://mingw-users.1079350.n2.nabble.com/Status-of-MSYS2-td7582391.html

We pick up msys2 minimal dependencies to build ruby.
Need to set MSYSTEM=MINGW32 environment variable.

Default value of MSYSTEM is changed with msys2.
http://sourceforge.net/p/msys2/tickets/4/
Pass make options by MAKE_OPT environment variable

We can pass -jN to make for build speedup.
Replace mingw/gnuwin32 packages to msys2