Always expose LOCALBASE, not just when CROSS_TOOLCHAIN is defined

Instead of using which(1) to look for doxygen, look for it in <LOCALBASE>/bin .
$PATH gets mangled by make buildenv, etc so it's better to just be explicit
about the path if someone uses that for instance.

Differential Revision: https://reviews.freebsd.org/D4406 (part of a larger diff)
Reviewed by: emaste, Evan Cramer <eccramer@gmail.com>
Sponsored by: EMC / Isilon Storage Division