Run cmake-init and setup flake.nix and default.nix

cmake/prelude.cmake

@@ -0,0 +1,10 @@
+# ---- In-source guard ----
+
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
+  message(
+      FATAL_ERROR
+      "In-source builds are not supported. "
+      "Please read the BUILDING document before trying to build this project. "
+      "You may need to delete 'CMakeCache.txt' and 'CMakeFiles/' first."
+  )
+endif()