master 49419912f2: Port alignas definition to C23

[Paul Eggert]

branch: master
commit 49419912f20b3cf0131a40798b162cbb596d368e
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>

    Port alignas definition to C23
    
    * src/lisp.h (alignas): C23 is removing the __alignas_is_defined
    macro, so do not rely on it. Instead, do not define alignas if C23
    or later, or if C++11 or later.
---
 src/lisp.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/lisp.h b/src/lisp.h
index 2f73ba4c61..9710dbef8d 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -245,7 +245,8 @@ DEFINE_GDB_SYMBOL_BEGIN (EMACS_INT, VALMASK)
 DEFINE_GDB_SYMBOL_END (VALMASK)
 
 /* Ignore 'alignas' on compilers lacking it.  */
-#if !defined alignas && !defined __alignas_is_defined
+#if (!defined alignas && !defined __alignas_is_defined \
+     && __STDC_VERSION__ < 202311 && __cplusplus < 201103)
 # define alignas(a)
 #endif