Fix pkgconfig file to make #include <capstone/capstone.h> work

[ret2libc]

Your checklist for this pull request

• [-] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• [-] I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

The simple example in https://www.capstone-engine.org/lang_c.html does not work for me because the it can't find capstone/capstone.h .

This is because the pkgconfig file only adds -I{includedir}/capstone and not -I{includedir}.

Test plan

Just try to compile the code in the documentation page.

Closing issues

...

[XVilka]

Note, that this is controversial change that was done back and forth mutliple times in the past. So it should be checked carefully first why it was done. cc @kabeor @Rot127

[Rot127]

Please someone correct me if I am wrong. But don't we, without this change, get conflicts with other package's headers of the same name? Basically like #2316?

@aquynh @kabeor Any comments?

[ret2libc]

Yes indeed the thing is that "recently" it was made "mandatory" to use #include <capstone/xyz.h> but then the pkgconfig file does not actually produce the right cflags to make that work.

[Rot127]

Any reference to "recently" and the "mandatory" points? Because we should add the explanation to the commit message. So the discussion is decided once and for all :)

[ret2libc]

Any reference to "recently" and the "mandatory" points? Because we should add the explanation to the commit message. So the discussion is decided once and for all :)

I don't have any, sorry :D Just memory, so I could very well be wrong

[Rot127]

I would like to test the collision theory before merging this. So we can say with absolute authority that this is the only way to do it, from now on.
@ret2libc Would you mind doing this?

@grahamwoodward Can you test this one as well for you please?

[ret2libc]

I think it's slightly different from the collision you referenced. The pkg-config cflags are used when another package wants to depend on capstone (e.g. Rizin). AFAIR rizin fails to compile now with system capstone.

[aquynh]

ok so we just need to update the docs on the website, right?

[ret2libc]

ok so we just need to update the docs on the website, right?

No, the doc on the website uses capstone/capstone.h which is correct, but it does not use pkgconfig, so this bug is not hit. However people using libraries usually rely on pkg-config or cmake files to get the right cflags/libs.

[Rot127]

I am in favor for this one. Because the header filenames are pretty generic being more explicit from which package they are, is better IMHO.

[XVilka]

@kabeor @aquynh, what is your opinion on this one? It's better to merge this early to allow a few months of testing on various systems/distributions so it wouldn't make a surprise before the 6.0 release

[kabeor]

Thank you!