Automatically determine python and dot paths

[tstenner]

This simple fix finds python and dot on the current system path, so dot in /usr/bin is found automatically.

[ss23]

There are configurations that will give the webserver an empty PATH environment variable, which would mean for those people, this patch is worse than before.

Rather than do this and break more configurations, I think a better fix would be to check whether dot / python can be find with the default path, and if not, try system, else give an error at that point.

[tstenner]

Commit 787744f addresses this problem.
If the defaults aren't executable, they are being set with exec('which').

[alpha0010]

Too much auto-magic is not very portable. Webgrind currently displays a warning page if the executables are not available.

Closing.