Update to rake-compiler-dock-0.5.1, which adds Ruby-2.3 support for Windows annd makes --disable-rubygems unnecessary

are checked while development.

C++ DLLs built with i586-mingw32msvc and i686-w64-mingw32 are not compatible.

So this changes the build, so that we ship the ports directorys of both
compiler versions.

This fixes https://github.com/larskanis/fxruby/issues/22

This also fixes some build system checks, that should be for the target system.

Static linking all dependent libraries into each ruby version (from
1.8 to 2.2) would blow up the gem to >20MB.

Update cross compilation tasks to build x64 and x86 targets.

Before commit af543822 loading order of fox16.so and fox16.rb was irrelevant.
But now fox16.rb must be called, in order to load ruby files to the C++
extension. Renaming the C++ extension ensures, that fox16.rb is called first.

change build dependencies so that (cross) compile and gem tasks no longer need the explicit configure task

add tasks for downloading and cross compilation of libfox, fxscintilla and depending libraries for win32

By default it looks like rake-compiler doesn't modify the gem spec
appropriately when we're cross-compiling for multiple Ruby versions.
Added some lines to remove "lib/fox16.so" from the gem spec's list
of files to include, and then added "lib/1.8/fox16.so" and "lib/1.9/fox16.so"
back to the list, so that we get the correct result.