CACHEDIR.TAG: use CMake to configure this file

This ensures that IDEs will generate the file during configuration,
rather than relying on the use to run `init.py` (which confuses CLion
for some reason).

CMakeLists.txt

@@ -0,0 +1,5 @@
+configure_file (
+    "${CMAKE_CURRENT_SOURCE_DIR}/build/CACHEDIR.TAG.in"
+    "${CMAKE_BINARY_DIR}/CACHEDIR.TAG"
+    @ONLY
+)

init.py

@@ -88,7 +88,6 @@ if __name__ == '__main__':
         accumulated += [f'"-D{key}={val}"' for key, val in vars.items()]
         accumulated += args
     cmds = [
-        f"cp {os.path.join(self_dir, 'CACHEDIR.TAG.in')} CACHEDIR.TAG",
         f"cmake {source_dir} {' '.join(accumulated)}",
         ' '.join(build)
     ]