Use _CS_PATH as fallback PATH by default.

[rynbrd]

This allows python-exec to behave correct when called via /usr/bin/env - python.

This was discovered at tensorflow/tensorflow#10529.

This is easily reproducible by running it:

$ /usr/bin/env - python
python: unable to find executable in PATH.

The behavior of env is to call execvp which falls back to the output of confstr(_CS_PATH) when no PATH is set. This allows env - to work in most cases.

However, python-exec2c expects PATH to be set and does not fall back to confstr(_CS_PATH) when it isn't. This change enables this behavior by default when no FALLBACK_PATH is provided at build time.

I've tested this on my own Gentoo. The results are positive:

$ /usr/bin/env - python
Python 2.7.12 (default, Aug  2 2017, 11:41:16) 
[GCC 4.9.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

[mgorny]

Why would FALLBACK_PATH ever be empty? This looks like a broken install.

[rynbrd]

Couldn't tell you. Here's the ebuild for 2.4.4 that builds this without --with-fallback-path: https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-lang/python-exec/python-exec-2.4.4.ebuild

The 2.4.5 ebuild does set --with-fallback-path: https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-lang/python-exec/python-exec-2.4.5.ebuild

[rynbrd]

FYI my tests are without --with-fallback-path set during configure.

[mgorny]

That's because the fallback code has been added in 2.4.5. We need to stabilize this version.

[mgorny]

In any case, the current behavior is intentional and the bug you're trying to fix is already fixed in the current git.