The `std_cmake_args` variable already contains this flag, so it's redundant.
And it alone doesn't make the `make install` command install to the appropriate location. As I pointed out, stuff gets installed to `"#{prefix}/usr/share/osm2pgrouting"`.

The patch I've added fixed this by removing the hard-coded `/usr/share` prefixes, and replacing them with relative paths.
I'll try to replace it with `inreplace` later today.

Thanks