build: let old build system coexist with waf

Rename configure and the makefile, because otherwise it will severely
confuse users. Normally ./configure && make is the way to build, which
in our case would use the old build system.

./old-configure will actually create the Makefile, so "make" will still
work after ./old-configure is run.

Don't create config.h in the top-level directory, because the waf build
would pick it up and create a major confusing mess. Create it in
old_build/ instead, and add it as include path to the CFLAGS. config.mak
could stay in the root, but move it to old_build too for symmetry.
build: verify downloaded waf binary

I don't feel particularly good about executing automatically downloaded
files from the internet without verification. Verify the downloaded
executable with sha256sum, which is part of GNU coreutils, and should be
ubiquitous enough.

Now it should be impossible to run a downloaded file other than what the
hash stored in autogen.sh indicates, which in turn is bound to the git
commit hash.

If she256sum is not available, print a warning, and don't make waf
executable.
autogen.sh: print some instructions what to do next