doc: announce C11 requirement

Add a deprecation notice informing users that we will require a C11
compiler from 23.11 release onwards. This requirement was agreed by
technical board to enable use of newer C language features, e.g.
standard atomics. [1]

[1] https://mails.dpdk.org/archives/dev/2023-February/263516.html

Signed-off-by: Bruce Richardson <[email protected]>
Acked-by: Tyler Retzlaff <[email protected]>
Acked-by: Aaron Conole <[email protected]>
Acked-by: Jerin Jacob <[email protected]>
Acked-by: Thomas Monjalon <[email protected]>

doc/guides/rel_notes/deprecation.rst

@@ -11,6 +11,24 @@ here.
 Deprecation Notices
 -------------------
 
+* C Compiler: From DPDK 23.11 onwards,
+  building DPDK will require a C compiler which supports the C11 standard,
+  including support for C11 standard atomics.
+
+  More specifically, the requirements will be:
+
+  * Support for flag "-std=c11" (or similar)
+  * __STDC_NO_ATOMICS__ is *not defined* when using c11 flag
+
+  Please note:
+
+  * C11, including standard atomics, is supported from GCC version 5 onwards,
+    and is the default language version in that release
+    (Ref: https://gcc.gnu.org/gcc-5/changes.html)
+  * C11 is the default compilation mode in Clang from version 3.6,
+    which also added support for standard atomics
+    (Ref: https://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html)
+
 * kvargs: The function ``rte_kvargs_process`` will get a new parameter
   for returning key match count. It will ease handling of no-match case.