Building PyPy from source is a nightmare and the shipped top-level
Makefile is a joke:
- nothing configurable at top-level to do the 2 common operations:
  build & install;
- the main binary searches for the library at hardcoded and
  undocumented places relative to where it is installed;
- no option to exclude specific components ? (here, _tkinter);
- lack documentation to specify location of required libraries;
- nothing out-of-the-box to build with several cores (it's even
  strange that in 2020, the biggest part is single threaded and
  it can't be resumed; I'm curious to know how developers work).

I found many answers in the Debian packaging.

This commit creates a Makefile file for 2 reasons:
- slapos.recipe.cmmi (forked from hexagonit.recipe.cmmi)
  does not have options to specify a shell snippet that is
  executed only during the build step
- by splitting the build in several steps, it makes debugging
  easier when it does not have to rebuild from the beginning

The drawback is that:
- make recipes need to be indented with tabs,
  in addition to the buildout indent of 2 spaces;
- $ escaping is tricky.