Look for protobuf on custom paths then system

[dberzano]

• Enforce presence of protoc (the protobuf compiler) too
• Similar to ZeroMQ and Go fixes #510 and Look for nanomsg on custom paths then system #548

[dennisklein]

@dberzano I have no objections here, but just to give a headsup what will come soon: In #632 I already changed the behaviour of all find modules to also search in system paths. Also I introduced a CMake variable matching the exported variable in your alidist recipes. So, in this case instead of ProtoBuf_DIR I used PROTOBUF_ROOT.

[dberzano]

Thanks @dennisklein I saw your PR. Just to be clear:

CMake's de facto convention (i.e. for most FindPackageNames) is to pass PackageName_DIR variables, not PACKAGENAME_ROOT ones, and to rely on what you explicitly pass (e.g. with -D) as opposed to picking things from the environment.

What I think is that:

• ProtoBuf_DIR and all other PackageName_DIR should be kept as they are and not deprecated in favour of PACKAGENAME_ROOT ones.
• No PACKAGENAME_ROOT variable should be picked from the environment when configuring a project (following the principle of "explicit is better than implicit").

My two cents :-) I'll keep an eye on #632!

[dennisklein]

(...) as opposed to picking things from the environment.

No PACKAGENAME_ROOT variable should be picked from the environment when configuring a project (following the principle of "explicit is better than implicit").

I fully agree with the notion, but it might not be possible to support all the needs of our clients by not picking up dependency hints from the environment. I will try to make everyone aware of this issue again and we will see, if I can convince them to support your requirement.

CMake's de facto convention (i.e. for most FindPackageNames) is to pass PackageName_DIR variables

I was not aware of this convention, thx. I see no reason not to support both variable naming schemes. If people agree to waive environment pickups, I would probably drop the aliBuild variable naming scheme again from #632 and stick with the CMake convention only.