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()
CMake: fix lib hack for OSX

External libraries (i.e. libraries that are not included in the repo)
should be passed to target_link_libraries() by using variables set
by find_package() and find_library(). Else, the linking will fail
if the libraries are not found in the default library search paths
which can be different on every platform.