[cbuild2cmake] Catch `executes` multiple rules

[brondani]

Multiple executes having one or more common outputs can lead to the following error:

ninja: error: build.ninja:427: multiple rules generate D:/examples/MDK-Middleware/Examples/Network/HTTPS_Server/Web.c

For instance this can be derived from the iteration of cproject level executes nodes for several build and target types.

cbuild2cmake should catch this situation and throw an explanatory error message avoiding further processing.