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's make specification and present in at least GNU make and BSD
make.

Problem most recently brought to attention by Adrian Sampaleanu.
Merge pull request #77 from earl/fix-recursive-make

Fix recursive make calls - makes sense for various other systems.