CMake: add stuff from CMAKE_PREFIX_PATH to PKG_CONFIG_PATH.

CMake: add stuff from CMAKE_PREFIX_PATH to PKG_CONFIG_PATH.

Pull in some code from CMake 3.12.4's FindPkgConfig.cmake to arrange
that, when running pkg-config, directories from CMAKE_PREFIX_PATH are in
the PKG_CONFIG_PATH environment variable.

We do this because we want to make sure that, if CMAKE_PREFIX_PATH is
set, and it points to a directory that contains a libpcap that has a .pc
file installed, we get that .pc file, but we don't yet want to require a
minimum of CMake 3.1 or later (CMake 3.1 and later do that
automatically) because there might be some long-term support OS version
that comes with an older version of CMake.