Remove local mirror for build files.

It is irrelevant in an open source context anyway, and since all
distfiles are cached, the load on the servers is really not that high.

At the same time make sure we don't pass too few parameters to
pkg-get-src in case there are no distfiles (libgcc).
Redmine #6766: Don't rely on "which" exit code.

On HPUX it always returns 0. What we should look at is the output.
Redmine #6766: Add /usr/contrib/bin to PATH, for HPUX.

Some tools, like gzcat, reside there.
Redmine #6766: Reuse Solaris INT_MAX fix for HPUX as well.
Redmine #6766: Switch the last platform, HPUX, to LMDB as well.