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.

1 files changed, 6 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index bd751f2..ceb28b5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-08-03  Zack Weinberg  <zackw@panix.com>
+
+	* include/features.h (__glibc_clang_prereq): New macro.
+	* misc/sys/cdefs.h (__glibc_clang_has_extension)
+	(__attribute_deprecated_msg__): New macros.
+
 2016-08-03  Joseph Myers  <joseph@codesourcery.com>
 
 	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_BFP_EXT): New