libffi and python: need threads support

libffi depends on pthreads.

Python depends on libffi (it can provide a builtin libffi, but also depends on
pthreads). Thus this patch also disable Python support if toolchain is compiled
w/o treads support.

Fixes http://sysmic.org/~jezz/results/204099dd

:

../src/closures.c:119:21: fatal error: pthread.h: No such file or directory

[Peter: fix comment dependencies, add python3]
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>