Add header search paths to protobuf-c++ spec

[Niranjan-B]

The Problem:
Consuming Protobuf runtime in Swift via "Protobuf-C++ podspec" fails compilation as it isn't able to resolve the header imports.

Solution:
Adding "HEADER_SEARCH_PATHS" to the "pod_target_xcconfig" flag will help "Protobuf-C++" pod discover the required headers.

NOTE:- Current obj-c builds are linting "Protobuf.podspec" and not "Protobuf-C++.podspec", hence they are all green.

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[thomasvl]

Core protobuf-team manages this podspec, so I'm not sure if this would be a good addition or not.

[deannagarcia]

Thanks for the contribution! Can you explain why this file needs the header argument but Protobuf.podspec doesn't?

[Niranjan-B]

Thanks for reviewing @deannagarcia.

I think it is to do with the way headers are imported in obj-c files (obj-c runtime is shipped using Protobuf.podspec) vs c++ files (c++ runtime is shipped using Protobuf-C++.podspec).

Protobuf.podspec -
I think we don't need header search paths since the imports do not use angled brackets. This tells the compiler to look for headers in the same directory first.
Eg. of an import - #import "GPBAny.pbobjc.h" in GPBAny.pbobjc.m

Protobuf-C++.podspec -
Since the imports in C++ files includes angled brackets (Eg. #include <google/protobuf/message_lite.h>), clang searches for those headers in the system directory or include path list. Since we want the compiler to use headers that are shipped with the framework, we would have to specify the directory in the include path list. This is achieved using the HEADER_SEARCH_PATHS flag.

[deannagarcia]

Thanks for the explanation! Once the tests pass, I will get this merged.

[Niranjan-B]

Hi @deannagarcia, do you have an ETA on when this commit would be picked up for the next release, please?

[Niranjan-B]

Hi @deannagarcia, sorry for tagging you again. I've been waiting quite some time for this fix. Could this be included as part of v21.2 Please?

cc: @thomasvl, @esorot

[esorot]

Apologies, but this was missed in the v21.2 build. This will have to wait for our next release.

[Niranjan-B]

Hi @esorot, When would the next release be out? Or could I cherry-pick this commit?