When installing, set rpath relative to executables

[s3rvac]

Currently, after make install, rpath of executables is set to an absolute path into the installation directory:

$ readelf fileinfo -d | grep rpath
0x000000000000000f (RPATH)     Library rpath: [/home/XXX/projects/retdec-install/lib]

This prevents us from moving the installation directory into a different location, which is useful e.g. when the installation is performed on one machine but we want to run the executables on a different machine. A better way would be to set rpath relative to executables. With GCC/Clang on Linux, this can be done by passing -Wl,-rpath,"\$ORIGIN/../lib".

Notes:

• CMake RPATH handling
• The solution should ideally work on both Linux and macOS. However, when comes to dynamic linking, both operating systems have their specifics, so this needs to be taken into account.

[s3rvac]

Implemented in 3c76114.