Instead of using custom variable to search for installed libraries
we can rely on the standard `CMAKE_PREFIX_PATH` [1]

[1]: https://cmake.org/cmake/help/v3.8/variable/CMAKE_PREFIX_PATH.html#variable:CMAKE_PREFIX_PATH

Setting the properly the JMALLOC_LIBRARIES variable into the find module
made possible to avoid the conditonal inclusion of the library

When rocksdb is build with jemalloc linking with adafs was failing