pkg_get_variable: support CMAKE_PREFIX_PATH
Not unlike Sam's patch in https://cmake.org/Bug/view.php?id=15805 but avoids changing behavior in internal case. Passes his test case, too.
@ben.boeckel worked with Sam a bit and was going to do a test case, so maybe he can review my test case and implementation.
Fixes #15805 (closed)
Edited by Dan Kegel