Prior to this commit, our Makefiles assumed the name of the make program
was simply "make".
Since we use GNU make extensions, this isn't always the case --
operating systems like FreeBSD come with their own implementation named
make which can be incompatible with the extensions used in by tinygo.
To run a compatible make on some of these systems, we have explicitly
call GNU make by running gmake.
This commit changes references to the command "make" to "$(MAKE)" which
is a variable that contains the name of the executable invoked to
process the Makefile.
This allow the Makefiles to be uniformly processed by the same make
program.
