earl fix recursive make calls

Instead of hardcoding the name of the make command, use the predefined
$(MAKE) variable for recursive make invocations.

This fixes the build for environments where make is called something
else (like gmake, or mingw32-make). $(MAKE) is standardised in POSIX
make and present in at least GNU make and BSD make.

Problem most recently brought to attention by Adrian Sampaleanu.