Makefile: run pyflakes as 'pyflakes', not python `which pyflakes`

This funky invocation syntax was introduced in 2007 (commit 56ad518),
with a comment of "make pyflakes run faster". I no longer have any idea
why that might have been the case. It's time to simplify this, because
some of our buildslaves have pipsi-installed "pyflakes" on their $PATH,
which use a "python" that's different than the one on $PATH.