pgrouting: boost 1.56 compatibility

Fixes build issues with pgrouting described in #31428 and #31417 by applying @kbinani's fix from https://github.com/pgRouting/pgrouting/issues/274#issuecomment-56154086.

While I was here, I noticed pgrouting is not designed to install to its own rack; it installs directly into the postgres rack, which seems impolite. :japanese_goblin:

Witness: https://gist.github.com/anonymous/809a12bcda799e09e626#file-02-make-L209-L222