Perform multiple separate searches in order.  If ZLIB_ROOT is set search
it exclusively so it takes precedence over CMAKE_PREFIX_PATH.  This
allows a user to provide -DZLIB_ROOT=/path/to/zlib/prefix on the CMake
command line to tell it exactly where to find zlib.  Otherwise fall back
to a normal search.

Inspired-by: Andreas Schneider <asn@cryptomilk.org>