Use `GNUInstallDirs` in `CMakeLists.txt`

[buzz]

I'm maintaining an Arch PKGBUILD for Dethrace in the AUR.

The current build process required a small patch, which I've applied here: fix_install_dirs.patch. This patch adopts GNUInstallDirs for install paths, making the project more compliant with CMake conventions and easier for distro packaging.

For developers who want the original "install everything in ." behavior, CMake can still be invoked like:

$ cmake -B build -S . \
  -DCMAKE_INSTALL_PREFIX=. \
  -DCMAKE_INSTALL_BINDIR=. \
  -DCMAKE_INSTALL_DOCDIR=.