FindCImg.cmake: Propagate png and jpeg library paths properly

According to FindPkgConfig documentation, the ${prefix}_LINK_LIBRARIES expands
to a full path to the library. This is what we want to pass to
target_link_libraries() later, because this allows CMake to properly generate
-L and -l flags. Using just ${prefix}_LIBRARIES works on Linux, where libraries
are put in the default library search path, but fails on FreeBSD.

This change does not fix the case for STATIC_BUILD due to bug on the CMake side:
https://github.com/Kitware/CMake/commit/7e4adcac820d90bb431b374a07ee3d0c6cfe3f1f