cmake: use FULL install directory variants in pkg-config file

[getchoo]

Description

Instead of manually prepending the install prefix to CMAKE_INSTALL_<dir> (which may already be an absolute path), we can let cmake do it for us

See https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html#result-variables for documentation about this

Existing Issue(s)

[madebr]

This would re-introduce absolute paths and make a SDL binary package non-relocatable.

[OPNA2608]

Maybe https://github.com/jtojnar/cmake-snips/blob/master/README.md#concatenating-paths-when-building-pkg-config-files should be used to construct these values (or just cmake_path if CMake minimum 3.20 is acceptable). That way:

• ${prefix} will be kept if the used CMAKE_INSTALL_*DIR variables are relative (which applies to the default values & the majority of build configs and use cases, I imagine), but
• if the CMAKE_INSTALL_*DIRs are already absolute, then they are taken as-is with the assumption that they're supposed to be non-relocatable

This is what has been accepted in fmtlib, for example: fmtlib/fmt#1657