src/CMakeLists.txt: Enable multiarch for the cmake files

src/CMakeLists.txt: Enable multiarch for the cmake files

The cmake files install path is hard coded which gets in the way of
enabling multiarch.

Fix this by respecting the standard CMAKE variable used to specify
library install location. This variable gets used during package
building to install the cmake files to multiarch aware location.

Gbp-Pq: Name 0003-src-CMakeLists.txt-Enable-multiarch-for-the-cmake-fi.patch