Add utility macros for clang detection, and deprecation with messages.
There are three new macros added 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.
The first two are used to define the third. The third will be used
in subsequent patches.
* include/features.h (__glibc_clang_prereq): New macro.
* misc/sys/cdefs.h (__glibc_clang_has_extension)
(__attribute_deprecated_msg__): New macros.