Make the Makefiles more portable

Make the Makefiles more portable

	- BSD make doesn't support the GNU extensions, and in that
	systems the GNU make is called gmake. Seeting the value of
	MAKE directly to make break the compilation in that systems.

	- In the same way, in BSD systems yacc is not aliased to byacc,
	and that explicit rule is not needed at all because POSIX
	make already has a default rule to generate c files from y
	files.