Update adding_packages.md

docs/development/adding_packages.md

@@ -10,14 +10,21 @@ The recipe format is described in the [rattler-build recipe format](https://gith
 Adding cmake based packages is easy. Usually it is enough to replace the `cmake` command with the `emcmake` command and
 `make` with `emmake` (see the [emscripten documentation](https://emscripten.org/docs/compiling/Building-Projects.html#integrating-with-a-build-system) for more details).
 
-To build a shared library with CMake, one needs these additional flags:
+To build a shared library with CMake, one needs  flags.
+These flags can be put into a `.cmake` file:
 ```CMake
+# overwriteProp.cmake
 set_property(GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS TRUE) # does not need to be global :)
 set(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-s SIDE_MODULE=1")
 set(CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS "-s SIDE_MODULE=1")
 set(CMAKE_STRIP FALSE)  # used by default in pybind11 on .so modules # only for needed when using pybind11
 ```
 
+This can be passed to the as command line argument to cmake
+```
+# CLI
+... -DCMAKE_PROJECT_INCLUDE=overwriteProp.cmake
+```
 
 
 **Example recipes**: