Uses/localbase.mk: Set CMAKE_PREFIX_PATH.

When USE'ing localbase.mk, make sure CMake is also aware that it is supposed
to give preference to ${LOCALBASE} when looking for files and libraries.

This is going to be a requirement once CMake is updated to 3.3.x, as
starting with this version it will by default use the PATH environment
variable (stripping the "/bin" or "/sbin" parts of each entry) to determine
where to find files and libraries. Since in most cases /usr will come before
/usr/local, it will find base's libarchive and fail at the configuration
stage on older FreeBSD releases.

Approved by: portmgr (antoine)
Differential Revision: https://reviews.freebsd.org/D3361