First check whether 2geom is installed system-wide

This allows distributions to ship 2geom separately from Inkscape, and to not build it again while building Inkscape itself.

This also allows developers to not have to clone the 2geom submodule if they don’t plan on working on it.

I’ve tested this MR on ArchLinux with the lib2geom-git package installed, Inkscape built and linked just fine, and at runtime I can see it load lib2geom.so:

lsof -p $(pidof inkscape) | grep 2geom
inkscape 91234 linkmauve  mem       REG               0,24          361152 /usr/lib/lib2geom.so.1.0.0 (path dev=0,27)