extensions/gnome: Add sdk include directories to C_INCLUDE_PATH

[3v1n0]

The sdk provides a base /usr/include path that should be checked when an including from the default paths.

Without this, when a file is including something such as

  #include <xkbcommon/xkbcommon.h>

that is provided by /snap/gnome-46-2404-sdk/current/usr/include, the compiler won't look for such path at all.

There's no need to fallback to default /usr/include because such path is always included anyways by gcc (at least when that's compiled with such prefix).

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[mr-cal]

Is there any risk of this breaking existing core24 snaps using the gnome extension?

[medubelko]

Doc changes LGTM.

[3v1n0]

Is there any risk of this breaking existing core24 snaps using the gnome extension?

The snaps no, builds it may... But well at rebuild time adjustments can be done.

However we're using it already in the sdk

[lengau]

The snaps no, builds it may... But well at rebuild time adjustments can be done.

This seems risky then. Our user contract is that an unchanged file should continue to build (as much as is in our control). There's an exception for stuff that's marked as experimental, but this extension is already marked as stable for core24.

The final call right now is @bepri's, but I think I'd rather add this for core26+

[bepri]

I think I agree with @lengau - I don't think it matters if it's an easy fix, needing a fix to a project file without a major version bump to Snapcraft is bad behavior. Another exception I've seen is for bugfixes, but this doesn't seem like a bug to me either.

I say this should be delayed until Snapcraft 9/core26.