use $(MAKE) variable in main Makefile

Using special $(MAKE) variable has some benefits over explicitly calling
'make'.
When using this variable, make knows automatically to pass some of its
arguments to lower-level invocations.
Among other features, this enables multi-threaded building from top-dir.
Merge pull request #78 from janisozaur/make-fixes

use $(MAKE) variable in main Makefile