Thanks for your help.
I have tried this, unfortunately it doesn't work, i'm guessing because of how cmake generates the Makefile (I've never worked with cmake before).

Doing `make install DESTDIR=/usr/local/Cellar/osm2pgrouting/2.0.0`, resulted in the program being installed to `/usr/local/Cellar/osm2pgrouting/2.0.0/usr/share/bin/osm2pgrouting`, and some other files to `/usr/local/Cellar/osm2pgrouting/2.0.0/usr/share/osm2pgrouting/<file>`.
So it's appending the /usr/share part...

I've tried the same with PREFIX, or even with the above DESDIR and an empty PREFIX, but it seems the PREFIX option has no effect.

Eventually I managed to get it done by patching the CMakeLists.txt file, and replacing the hard-coded references to '/usr/share'. 
Hope this is acceptable, not sure whether this is the recommended practice...
Thanks