CMake: convoluted use of paths

[rouault]

We should likely use GNUInstallDirs in a more simple way rather than the whole logic

include(ProjInstallPath)
set(BINDIR "${DEFAULT_BINDIR}"
  CACHE PATH "The directory to install binaries into.")
set(LIBDIR "${DEFAULT_LIBDIR}"
  CACHE PATH "The directory to install libraries into.")
set(DATADIR "${DEFAULT_DATADIR}"
  CACHE PATH "The directory to install data files into.")
set(DOCDIR "${DEFAULT_DOCDIR}"
  CACHE PATH "The directory to install doc files into.")
set(INCLUDEDIR "${DEFAULT_INCLUDEDIR}"
  CACHE PATH "The directory to install includes into.")
set(CMAKECONFIGDIR "${DEFAULT_CMAKEDIR}"
  CACHE PATH "Parent of the directory to install cmake config files into.")

[mwtoews]

Closed by #3150