I figured it out. I had a typo in my cmake config. Sorry for the noise.
My final working config is:
cd MobilityDB
export PGVER=14
export PGPATH=/usr/lib/postgresql/${PGVER}
rm -rf build
mkdir build
cd build
cmake -DPOSTGRESQL_PG_CONFIG=${PGPATH}/bin/pg_config ..
make
sudo make install
Thanks,
Regina