Add "inc" to HEADER_FILES_EXTENSIONS

[mozeryansky]

We have created a podspec for a c++ project, but they use ".inc" for extensions to their header files. Can we either add ".inc" or make a way to specify additional extensions for headers.

Here's a relevant conversation: CocoaPods/CocoaPods#3708

[dnkoutso]

This should be fairly easy to do https://github.com/CocoaPods/Xcodeproj/blob/master/lib/xcodeproj/constants.rb#L419

I read your other reply, historically this list doesnt change that often (last time it was changed was December 13th 2017).

I believe the burden of adding the ability to customize these extension is actually higher than the fixed list.

The PR should be simple and we can get it merged quickly.

[mozeryansky]

I agree that the right solution would not be appending to that list, but to allow custom extensions. Common extensions should be in that list.

[mozeryansky]

Ha there already is a PR for adding ".cc"

#514

[dnkoutso]

Seems like the .cc one went through a different path.

[mozeryansky]

".inc" was already added! #598

[dnkoutso]

@mozeryansky I just did this in this PR? what do you mean?

[mozeryansky]

Oh, yes. Very fast. Thanks.