PORTS_MODULES fix

I have recently tried the PORTS_MODULES knob, and found a problem. The
ports tree searches for some dependencies by finding a binary in PATH,
and that fails since by default /usr/local/ isn't there. The attached
patch fixes that problem.

It would be more robust to use PREFIX there instead of /usr/local
explicitly, but I'm not sure how to unravel the mk maze to get that
value. If anyone has a suggestion for that, I'd be happy to include it.

Any objections to making this change?

Doug

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