use environment variables to override default executables used by make

python may not exist (on some of the BSDs) or may be python 3 (on
archlinux). We can now choose an executable by setting the environment
variables PYTHON (or calling make PYTHON=...).

While I'm here, do the same for m4 and cython (environment variables
M4 and CYTHON)

Makefile

 # This file is renamed to "Makefile.ext" in release tarballs so that setup.py won't try to
 # run it.  If you want setup.py to run "make" automatically, rename it back to "Makefile".
 
+PYTHON ?= python
+CYTHON ?= cython
+M4 ?= m4
+
+
 all: gevent/gevent.core.c gevent/gevent.ares.c
 
 gevent/core.pyx: gevent/core_.pyx
-	m4 -P gevent/core_.pyx > core.pyx && mv core.pyx gevent/
+	$(M4) -P gevent/core_.pyx > core.pyx && mv core.pyx gevent/
 
 gevent/gevent.core.c: gevent/core.pyx gevent/libev.pxd
-	python util/cython_ifdef.py -o gevent.core.c gevent/core.pyx
+	$(PYTHON) util/cython_ifdef.py -o gevent.core.c gevent/core.pyx
 	echo                          >> gevent.core.c
 	echo '#include "callbacks.c"' >> gevent.core.c
 	mv gevent.core.* gevent/
 
 gevent/gevent.ares.c: gevent/*.pyx gevent/*.pxd
-	cython -o gevent.ares.c gevent/ares.pyx
+	$(CYTHON) -o gevent.ares.c gevent/ares.pyx
 	mv gevent.ares.* gevent/
 
 clean:
 	rm -f gevent.core.c gevent.core.h core.pyx gevent/gevent.core.c gevent/gevent.core.h gevent/core.pyx
 	rm -f gevent.ares.c gevent.ares.h gevent/gevent.ares.c gevent/gevent.ares.h
+
+.PHONY: clean all