make shotwell's facedetection code compile with opencv 4

Patch adapted from upstream:

https://gitlab.gnome.org/GNOME/shotwell/-/commit/f54094594016359bbafd0be44a53ea28c8968593

At the same time, linking against opencv 4 fails when compiling shotwell
with gcc. As a stopgap measure, disable option OPENMP (which currently
forces use of gcc, but work is underway to switch that to llvm (see
PR 252379). I'll revisit OPENMP once 252379 has landed).