Add Visibility API for gcc/clang toolsets

[tdesveauxPKFX]

No description provided.

[samsinsane]

This would be much better implemented as an API instead of a flag. We're avoiding adding new flags, they start off as binary but eventually cease being binary. Also, when implementing as an API please consider Clang.

[tdesveauxPKFX]

It seems that Clang copy all cxx flags from gcc here.

I also added the other possible values from what running gcc -fvisibility=x displayed.

And I'm wondering about removing -fvisibility-inlines-hidden from this. It could be misleading in the present state.

[samsinsane]

Thanks for making those changes! I did some basic reading of what -fvisibility-inlines-hidden does, does it add anything to -fvisibility=hidden? It didn't look like it, but I just skimmed the GCC wiki. I'm happy for this to get merged in, but I'll leave it for someone else to review and add their opinion on -fvisibility-inlines-hidden. Thanks again!

[tdesveauxPKFX]

From what I read, it seems indeed that visibility-inlines-hidden can be useful on it's own only.
I will setup an other API for it.

[samsinsane]

Sounds good!

[tdesveauxPKFX]

Inlines visibility moved to another PR (#1049)

[tdesveauxPKFX]

@tvandijck why did you close this?
Although I move inlines visibility to another PR, the visibility API is still useful.

[samsinsane]

I'm going to assume this was an accident.

[tvandijck]

No, not an accident, just confused.. It said "Inlines visibility moved to another PR (#1049)", so I assumed this one was no longer needed...

At the very least the PR should be renamed to describe what it is now doing though, considering it no longer "Adds VisibilityHidden to flags"...

[tdesveauxPKFX]

Indeed, I should have updated the title with the changes I made. My bad.

[tvandijck]

merge conflict. other then that, ready to merge.

Merge conflict has been cleared.