math/saga: Configure not finding OpenCV headers
- Let pkg-config find the OpenCV 3.x headers after change in sources[1]
- Convert sed replacement to proper patch file
- While here, remove unneeded OpenMP post-patch
[1] https://sourceforge.net/p/saga-gis/code/ci/1a4b6df792186480953f4fb20474f16b328a9a5c/
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D26645