[cmake] root wrapper

allow the existence of a `CMakeLists.txt` file at root,
for easier integration with other projects expecting this file at root.

Existing integration point, within `build/cmake/`, still works as expected.

build/cmake/README.md

@@ -7,6 +7,15 @@ variables.
 
 ## How to build
 
+You can configure the project from the repository root thanks to the forwarding
+`CMakeLists.txt`:
+```sh
+cmake -S . -B build-cmake
+cmake --build build-cmake
+```
+The historical workflow that starts configuration from `build/cmake` continues
+to work as described below.
+
 As cmake doesn't support command like `cmake clean`, it's recommended to perform an "out of source build".
 To do this, you can create a new directory and build in it:
 ```sh