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.