I just realised a further problem that means that even with the above issue fixed then the pcl pkg-config files are still unusable because pcl's headers also depends on Eigen 3 (and so the pkg-config --cflags also need to have -I/usr/include/eigen3 otherwise the pcl headers aren't usable.
Workaround:
sudo sed -i 's/Requires:/Requires: eigen3 /g' /usr/lib/x86_64-linux-gnu/pkgconfig/pcl_common-1.8.pc
I just realised a further problem that means that even with the above issue fixed then the pcl pkg-config files are still unusable because pcl's headers also depends on Eigen 3 (and so the pkg-config --cflags also need to have -I/usr/ include/ eigen3 otherwise the pcl headers aren't usable.
Workaround:
sudo sed -i 's/Requires: /Requires: eigen3 /g' /usr/lib/ x86_64- linux-gnu/ pkgconfig/ pcl_common- 1.8.pc