osm2pgrouting compile instructions needs updating for cmake flags

[robe2]

Now that @sanak put in the changes for postgres install. I notice I have to add:

-DPOSTGRESQL_INCLUDE_DIR:PATH=${PGPATH}/include

To my cmake call. This is good since it does allow me to easily switch which postgres to use without stuffing it in my PATH variable as I had before. However it should be documented as an argument to pass cmake.

In general I think that Readme.md needs some work and should list all the CMAKE flags. I'll try to update that in my branch.

[sanak]

@robe2 Okay, thanks for information. I will check it later.