Add allow_symlinks_before_srcdir config setting

Add allow_symlinks_before_srcdir config setting

can be used to avoid a security check that is a good safe default, but
problimatic overkill in some situations.

I decided to underdocument this, because the option looks ugly, and I don't
want people randomly turning it on because it looks like a good idea. So if
you need it, you'll get an error message mentioning how to fix it.