Apparently sysconfig.get_config_var() is already called in site.py anyway,
so in principle I'm fine with using it here.

In the stdlib it seems to be the first use outside site.py though, and
site.py can be disabled by using "python -S", so perhaps we should be
extra careful, set UNIX_SHELL unconditionally to "/bin/sh" and move
the Android conditional inside a try/except.


But perhaps I'm being irrational here.  Victor, what do you think?