switch the build system to waf

This commit adds a new build system based on waf. configure and Makefile
are deprecated effective immediately and someday in the future they will be
removed (they are still available by running ./old-configure).

You can find how the choice for waf came to be in `DOCS/waf-buildsystem.rst`.
TL;DR: we couldn't get the same level of abstraction and customization with
other build systems we tried (CMake and autotools).

For guidance on how to build the software now, take a look at README.md
and the cross compilation guide.

CREDITS:
This is a squash of ~250 commits. Some of them are not by me, so here is the
deserved attribution:

 - @wm4 contributed some Windows fixes, renamed configure to old-configure
   and contributed to the bootstrap script. Also, GNU/Linux testing.
 - @lachs0r contributed some Windows fixes and the bootstrap script.
 - @Nikoli contributed a lot of testing and discovered many bugs.
 - @CrimsonVoid contributed changes to the bootstrap script.