Fix math/kig build failures because it can't find Boost::Python.

This is another bit of fallout from the change in CMake 3.11.0 which
respects CMAKE_REQUIRED_LIBRARIES in check_headers. It took a while
for us to be able to reproduce the problem, though.

Submitted by: tcberner
Reported by: pkg-fallout
Reviewed by: adridg
Approved by: tcberner (mentor, implicit)