Current `pkgconfig` configuration file installation path and its
contents assume libraries are installed under `/usr/lib` and this is not
always the case, for instance `/usr/lib64` is quite possible under
Gentoo Linux.
Thus use the `CMAKE_INSTALL_LIBDIR` variable instead of a hardcoded
`lib`.
Depending on the OS the lib dir can vary, on Fedora for instance it is
"${prefix}/lib64". Instead of hard-coding the directory name, let CMake fill
this variable for us.
Patrice Levesque
Georgi Gerganov
Philippe Normand