Fixed locating cryptopp in system locations

if(\<var\>) is true as long as \<var\> is defined, so I changed the conditional to check for existence of the ../cryptopp562 directory instead.

find_path wasn't updating CRYPTOPP_INCLUDE_DIR, and find_library wasn't updating CRYPTOPP_LIBRARIES, so I explicitly unset them first. I suspect this has to do with the variables being in cache or not, but I really don't know enough CMake to say for certain. If crypto++ isn't anywhere at all, the behavior should be as expected.

These changes properly located my crypto++ library in /usr/include/cryptopp and /usr/lib under Arch Linux and CMake 2.8.12.1.