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.

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

include/features.h

@@ -138,13 +138,13 @@
 # define __KERNEL_STRICT_NAMES
 #endif
 
-/* Convenience macros to test the versions of glibc and gcc.
-   Use them like this:
+/* Convenience macro to test the version of gcc.
+   Use like this:
    #if __GNUC_PREREQ (2,8)
    ... code requiring gcc 2.8 or later ...
    #endif
-   Note - they won't work for gcc1 or glibc1, since the _MINOR macros
-   were not defined then.  */
+   Note: only works for GCC 2.0 and later, because __GNUC_MINOR__ was
+   added in 2.0.  */
 #if defined __GNUC__ && defined __GNUC_MINOR__
 # define __GNUC_PREREQ(maj, min) \
 	((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
@@ -152,6 +152,17 @@
 # define __GNUC_PREREQ(maj, min) 0
 #endif
 
+/* Similarly for clang.  Features added to GCC after version 4.2 may
+   or may not also be available in clang, and clang's definitions of
+   __GNUC(_MINOR)__ are fixed at 4 and 2 respectively.  Not all such
+   features can be queried via __has_extension/__has_feature.  */
+#if defined __clang_major__ && defined __clang_minor__
+# define __glibc_clang_prereq(maj, min) \
+  ((__clang_major__ << 16) + __clang_minor__ >= ((maj) << 16) + (min))
+#else
+# define __glibc_clang_prereq(maj, min) 0
+#endif
+
 /* Whether to use feature set F.  */
 #define __GLIBC_USE(F)	__GLIBC_USE_ ## F
 

misc/sys/cdefs.h

@@ -77,6 +77,15 @@
 
 #endif	/* GCC.  */
 
+/* Compilers that are not clang may object to
+       #if defined __clang__ && __has_extension(...)
+   even though they do not need to evaluate the right-hand side of the &&.  */
+#if defined __clang__ && defined __has_extension
+# define __glibc_clang_has_extension(ext) __has_extension (ext)
+#else
+# define __glibc_clang_has_extension(ext) 0
+#endif
+
 /* These two macros are not used in glibc anymore.  They are kept here
    only because some other projects expect the macros to be defined.  */
 #define __P(args)	args
@@ -249,13 +258,24 @@
 # define __attribute_noinline__ /* Ignore */
 #endif
 
-/* gcc allows marking deprecated functions.  */
+/* Since version 3.2, gcc allows marking deprecated functions.  */
 #if __GNUC_PREREQ (3,2)
 # define __attribute_deprecated__ __attribute__ ((__deprecated__))
 #else
 # define __attribute_deprecated__ /* Ignore */
 #endif
 
+/* Since version 4.5, gcc also allows one to specify the message printed
+   when a deprecated function is used.  clang claims to be gcc 4.2, but
+   may also support this feature.  */
+#if __GNUC_PREREQ (4,5) || \
+    __glibc_clang_has_extension (__attribute_deprecated_with_message__)
+# define __attribute_deprecated_msg__(msg) \
+	 __attribute__ ((__deprecated__ (msg)))
+#else
+# define __attribute_deprecated_msg__(msg) __attribute_deprecated__
+#endif
+
 /* At some point during the gcc 2.8 development the `format_arg' attribute
    for functions was introduced.  We don't want to use it unconditionally
    (although this would be possible) since it generates warnings.