Small refactoring for the build system

[skosukhin]

• fixes the parallel building in the tests directory;
• switches to the recommended way of calling make recursively;
• gives the users control over the maximum number of make processes (generally, -j without a value gives worse performance than with the value set to the number of virtual cores on a machine; it's also not very nice towards other users to spawn the unlimited number of processes on a login node);
• makes the check recipes honour the PYTHON environment variable when locating the Python interpreter (the python executable is not generally available and the users might want to set it either to python3 or to the path to the executable in a virtual environment).

Depends on #265

[RobertPincus]

I merged #265. Let me know when this PR is ready, @skosukhin

[skosukhin]

@RobertPincus this is ready.