Log Message:
Regularize path subsitution and use PREFIX

Use PREFIX rather than LOCALBASE.  What matters is where this packages
prefix is, not anything else.

Substitute all paths the same way, assigning to sh variables in one
place, alphabetically, and then using them.  Sort list of substituted
variables alphabetically also, so it's easier to review the code.

No functional change for any reasonable configuration.

Based on a suggestion by J. Lewis Muir on pkgsrc-users.