Fix build with LOCALBASE is not /usr/local.

[bdrewery]

Test Plan

Built llvm40 with a different PREFIX + LOCALBASE. It failed
before since it was using -I/usr/local/include and -L/usr/local/lib
rather than my supplied LOCALBASE (where existing headers/libs are).

[brooks]

Do we need a version bump or are people with non-default PREFIX values on their own?

[bdrewery]

In D10859#225031, @brooks wrote:

Do we need a version bump or are people with non-default PREFIX values on their own?

Since it is a build fix, no revision bump is needed. It would not build otherwise. It's also a rare thing for people to do this, but still proper to support it.