Add libint support

[adelavais]

Hello! I was working on adding internationalization to a program and I observed that locale.h exists in core.stdc, but libintl.h doesn't. I think introducing this header would help D, as functions from locale.h and libintl.h are often used together, and it would bring internationalization into D.
I am aware that this PR is technically a WIP, but I need help in continuing and feedback if this is something worth pursuing in D.

[dlang-bot]

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Testing this PR locally

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[pbackus]

The core.stdc package is for standard C headers [1]. locale.h is a standard C header, but libint.h is not—it is part of GNU gettext. The appropriate place for this is a package on code.dlang.org, not druntime.

[1] http://port70.net/~nsz/c/c99/n1256.html#7.1.2

[12345swordy]

@WalterBright what do you think?

[Geod24]

I agree with @pbackus this should be a dub package. The only need that this could have in druntime was if we were using it. druntime itself is not a binding library.

Code wise, there's two issues:

1. Attributes are missing. This should be @system nothrow @nogc.
2. There's some long int here. For C long type, you should use c_long, as it's 4 bytes in 32 bits (unlike D's long).

[adelavais]

Thank you so much for your feedback! I made a dub package instead (https://code.dlang.org/packages/libintl) and I will close this PR.