CMake: remove unnecessary link_directories()

"Note that this command is rarely necessary. Library locations
returned by find_package() and find_library() are absolute paths.
Pass these absolute library file paths directly to the
target_link_libraries() command. CMake will ensure the linker finds
them."

http://www.cmake.org/cmake/help/v3.0/command/link_directories.html
Merge pull request #149 from toxeus/cleanupCMake

CMake: remove unnecessary link_directories()