[build] Define and use MAKE var instead of explicitly calling make

[JamiKettunen]

There are multiple make implementations and nothing guarantees that the make executable is gmake, which is what this Makefile requires.

As a practical example https://chimera-linux.org/ uses bmake as the default make while also having gmake easily available for projects needing it.

My packaging of this project where this is used: JamiKettunen/cports@db1b003f

[spiiroin]

This is = how gmake documentation suggest recursive builds should be done.

[monich]

Thanks!

[Thaodan]

To late but defining make was redundant:
Make = make is part of IEEE Std 1003.1-2017 section Default Rules subsection Macros.

Also GNU make explains that $(MAKE) should always be used read:
https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html