The GNUInstallDirs module supports absolute paths for
CMAKE_INSTALL_LIBDIR and CMAKE_INSTALL_INCLUDEDIR. Some package managers
like Nix depend on this behaviour.
See https://github.com/NixOS/nixpkgs/issues/144170 for the nixpkgs
tracking issue.
The UHD CMake code re-implemented some of the features of GNUInstallDirs,
which is not necessary (this feature has been available in CMake for
longer than version 3.12). Here, we replace all hand-defined CMake
variables with their equivalents from GNUInstallDirs.
Short table of replacements:
INCLUDE_DIR -> CMAKE_INSTALL_INCLUDEDIR
RUNTIME_DIR -> CMAKE_INSTALL_BINDIR
LIBRARY_DIR -> CMAKE_INSTALL_LIBDIR
UHD_LIB_DIR -> CMAKE_INSTALL_LIBDIR
LIB_SUFFIX -> n/a (no longer necessary)
