Use system libdir when searching for dynamically loaded libraries #25

To dynamically load libraries, we need to identify the directory in which the library will be searched for.

Here are different ways to deal with multi-arch in individual distributions. Some consider only 64 × 32 bits and use lib64 × lib (RedHat, Slackware), some consider full multi-arch and use lib/x86_64-linux-gnu, lib/i386-linux-gnu, lib/arm-linux-gnueabihf,… (Debian, Ubuntu).

Currently, KDELIBSUFF was used, which assumed the '64' setting for lib64. However, this is not enough for a full multi-arch case. When searching for an SSL libraries, it is currently looking for all the lib/*-*-* directories, which can lead to finding a library that is for the other architecture.

The proposed patch uses the fact that CMake rules contain the default path for libraries. Instead of KDELIBSUFF, SYSTEM_LIBDIR is now used, which is set according to the CMake value. As a result, on fully multi-arch systems, a directory for the right architecture will be searched.