[PATCH 3/4] Add utility macros for clang detection, and deprecation with messages.

[Zack Weinberg]

On Fri, May 13, 2016 at 9:26 AM, Zack Weinberg <zackw@panix.com> wrote:
> Add three new macros to features.h and sys/cdefs.h:
>
>  * __glibc_clang_prereq: just like __GNUC_PREREQ, but for clang.
>  * __glibc_clang_has_extension: wraps clang's intrinsic __has_extension.
>    Writing "#if defined __clang__ && __has_extension (...)" doesn't work,
>    because compilers other than clang will object to the unknown macro
>    __has_extension even though they don't need to evaluate it.
>    Instead, write "#if __glibc_clang_has_extension (...)".
>
>  * __attribute_deprecated_msg__(msg): like __attribute_deprecated__, but
>    if possible, prints a message.

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