Fix multiarch install location for cmake package config

[gusnan]

Debians Multi-arch-hinter gives warnings for the cmake package config files installation directories - this should fix this. (For a comparison, checkout the folders /usr/lib/cmake/ and /usr/lib/x86_64-linux-gnu/cmake/ - there's a massive amount more using the x86_64-linux-gnu folder).

[SiegeLord]

Do you know which version of cmake this became available? The internet also says that this comes from include(GNUInstallDirs), which we don't explicitly include anywhere.

[gusnan]

Yeah, it looks to me it simply comes from GNUInstallDirs, so I guess the specific variable has been in cmake since that (which looks like around 3.0, somewhere around 2018). But you are right, I guess an include of GNUInstallDirs wouldn't hurt - I'll push a commit fixing this here.

[gusnan]

Some post on the cmake forums that argues for ${CMAKE_INSTALL_LIBDIR}/cmake/<packageName> here: https://discourse.cmake.org/t/what-should-the-destination-be-for-a-header-only-librarys-cmake-config-file/8473/2?u=gusnan