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
Add mingw64 4.8.2-rev0 x86 and x64 support
Only add DLDFLAGS for relevant DevKit's

Current GCC 4.8.x compilers add an implicit dependency to the libgcc
support library to msvcrt-rubyXYZ.dll. This overly complicates
installs and creates additional support concerns. Adding -static-libgcc
to configure's DLDFLAGS "fixes" this GCC behavior. Rather than
hard-coding DLDFLAGS for every build, set only when appropriate.

NOTE: while generic to 4.8.x, the DevKit version matching regex must
be updated to support future GCC versions if this GCC behavior persists.
Upgrade mingw64-{32,64}-4.8.2 to rev 1
Update mingw64-{32,64}-4.8.2 to rev2
Update mingw64-{32,64}-4.8.2 to rev3
Update mingw64-{32,64}-4.8.2 to rev4
Add mingw64-{32,64}-4.9.0 rev1 support
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
Add missing host option of x86_64 build
Add patch to MSYS2 based DevKit

The MSYS2 based patch utility embeds the manifest required to
prevent UAC issues on Windows systems.
Update mingw64-{32,64}-4.9.0 to rev2
Add mingw64-{32,64}-4.9.1 rev0 support