src/python-exec.c: utilize confstr(_CS_PATH) if PATH is unset

[floppym]

exec(3) from glibc says:

The execlp(), execvp(), and execvpe() functions duplicate the actions
of the shell in searching for an executable file if the specified
filename does not contain a slash (/) character. The file is sought in
the colon-separated list of directory pathnames specified in the PATH
environment variable. If this variable isn't defined, the path list
defaults to the current directory followed by the list of directories
returned by confstr(_CS_PATH). (This confstr(3) call typically returns
the value "/bin:/usr/bin".)

[floppym]

An alternative to this would be to hard-code a fallback PATH setting, probably based on a configure option.

[mgorny]

To be honest, I'm not convinced this is the best things to do. Citing POSIX:

If this environment variable is not present, the results of the search are implementation-defined.

This variable certainly isn't POSIX, and the glibc behavior is at best debatable. I certainly don't think it a good idea to use CWD like this.

[mgorny]

FreeBSD, Darwin:

If this variable is not specified, the default path
is set according to the _PATH_DEFPATH definition in <paths.h>, which is
set to ``/usr/bin:/bin''. For execvP(), the search path is specified as
an argument to the function. In addition, certain errors are treated
specially.

Solaris manpage doesn't mention fallback at all.

[mgorny]

So yeah, I would prefer that instead of emulating (bad) glibc behavior we'd set PATH to a sane value that would ensure consistent behavior on all systems.

[floppym]

Per our IRC conversation, I will open a new PR with a configure option for this instead.