cmake: [win32] Fix rocksdb cross compiling

cmake: [win32] Fix rocksdb cross compiling

In order to be able to cross compile rocksdb using mingw, we'll
have to do the following:
* pass on specified toolchain files
* pass the CMAKE_PREFIX_PATH. We'll have to replace the ";" separator
  with something else (we went with "!"), otherwise CMake won't be
  able to properly handle the command arguments
* pass on the received LZ4 lib location since it cannot be properly
  detected when cross compiling.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>