Use encoding magic comment to workaround codepages

There are problems with different codepages (`chcp`) that causes `dk.rb` not to work properly.

Use a magic comment to mark the script as `UTF-8` in an attempt to workaround those issues.

Ref #199.
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
Support mingw64-{32,64}-4.8.2 in Ruby20/Ruby21
Update mingw64-{32,64}-4.8.2 to rev2
Update mingw64-{32,64}-4.8.2 to rev3