Use stricter PATH during build

["Masao Uebayashi" <uebayasi%gmail.com@localhost>]

This makes pkgsrc build use stricter PATH during build; that is, don't
use PATH passed by user.

        http://www.NetBSD.org/~uebayasi/stricter-path.patch

This is experimental, but this shows interesting results.  Problems
I've found so far are:

- emulator/emulator.mk calls file(1) implicitly, which should be a TOOLS
- GNU configure usually uses uname(1) which is missed in TOOLS too

Masao