CMake build: please use the GNUInstallDirs module

As of 4.14.0, the CMake build system of Qt Creator hardcodes all the installation paths for binaries/libraries/etc in cmake/QtCreatorAPIInternal.cmake.

This suboptimal, as some of these paths must be tweaked when building on Linux and other Unix (e.g. *BSD) distributions, according to the distro policies: e.g. the multiarch library path for Debian-based systems, the multilib library path for RPM distros, and so on.

CMake already provides a way to set these paths according to the GNU coding standards:

https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html

This module also works non non-Linux Unix systems like *BSDs, and it also allows people building to change the paths.

I can understand this is not needed on Windows and macOS, so IMHO it could be OK to use it like:

if (APPLE)
elseif (WIN32)
else ()
  include(GNUInstallDirs)
  # use it
endif ()

Note: since QtCreatorAPI (which include()s ) is include()d before project(), then this breaks some of the bits that GNUInstallDirs sets (like ${CMAKE_LIBRARY_ARCHITECTURE}). Hence, that include() ought to be moved after project().