Rename MAKE environment variable to MAKE_COMMAND

Rename MAKE environment variable to MAKE_COMMAND

The environment variable MAKE has been replaced with MAKE_COMMAND,
because overriding MAKE variable also changed the definition of the MAKE
inside the Makefiles, and we want only a single wrapper around the whole
build process.

Previously, setting `MAKE` to `bear make` meant that `bear make` would
be run at every nested make invocation, which messed up the upcoming
automake transition as compile_commands.json would be generated in every
subdirectory instead of just having one central file at the top of the
build tree.