FindPkgConfig: also handle "-isystem" prefixes for include directories
This currently is just a rather naive "make it accept this". It does not pass on the system flag to CMake, to later tools will not see this as system directory and may e.g. generate additional compiler warnings.
Fixes: #20652 (closed)
Backport: release:HEAD^2
Topic-rename: FindPkgConfig-isystem
Edited by Brad King