[CMake] externalproject_add and generating sources

[Michael Ellery]

> On Jan 26, 2017, at 1:23 AM, David Jobet <david.jobet at free.fr> wrote:
> 
> Hello,
> 
> suppose I want to use protobuf and integrate it in my project with externalproject_add. (actually, I just have precompiled binaries and libs + header files, I don't have the full sources)
> Once the project has been 'built' (actually, installed by a custom rpm-like tool to a shared path), I can use the protoc compiler to generate protobuf c++ files (.pb.h and .pb.cpp files) from a .proto description file.
> I can add_dependencies so that the external project is built before projects depending on libprotobuf.a.
> 
> However, I don't know how to do the same thing with .pb.h and .pb.cpp files which use a .proto file and the protoc compiler.
> 
> make is fine with it, but ninja complains with an error like this :
> ninja: error: '/path_to_external_projects/protobuf/2.6.0.4/bin/protoc', needed by 'some_project/SomeFile.pb.h', missing and no known rule to make it
> 
> how can I tell ninja that protoc is going to be provided by externalproject ?
> If I use byproducts, ninja is happy, but "ninja clean" deletes /path_to_external_projects/protobuf/2.6.0.4/bin/protoc.
> I don't want that to happen since /path_to_external_projects/protobuf/2.6.0.4/bin/protoc is shared with other users.
> 

I’ve never used protobufs as an external project (I’ve always just done a one-time setup/install)…but is there a reason you can’t just use the standard finder to locate it?  https://github.com/Kitware/CMake/blob/master/Modules/FindProtobuf.cmake

-Mike