replace explicit make command with MAKE variable

Use $(MAKE) rather than explicitly naming `make' so that recursion
works when invoking `make' by a different name (e.g. `gmake').

See also GNU `make' manual [1].

[1] https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html
Merge branch 'fix/make-variable' of github.com:frasertweedale/r2d2b2g into frasertweedale-fix/make-variable
replace more explicit make command instances with MAKE variable