master 1e2bc1b: Port to Tiny C on x86-64

[Paul Eggert]

branch: master
commit 1e2bc1bbf47782df7f9b8051f33904432c55e6e4
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>

    Port to Tiny C on x86-64
    
    This allows Emacs to build on Ubuntu 21.10 x86-64 with
    ‘./configure --without-modules CC=tcc’.
    * configure.ac (GC_SETJMP_WORKS):
    Accept __x86_64__ as an alias for __amd64__.
    * src/lisp.h (alignas): Provide a no-op substitute.
---
 configure.ac | 1 +
 src/lisp.h   | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/configure.ac b/configure.ac
index c36dffd..0c23b60 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5426,6 +5426,7 @@ case $opsys in
 #if defined __i386__ || defined __sparc__ || defined __mc68000__ \
   || defined __alpha__ || defined __mips__ || defined __s390__ \
   || defined __arm__ || defined __powerpc__ || defined __amd64__ \
+  || defined __x86_64__ \
   || defined __ia64__ || defined __sh__
 /* ok */
 #else
diff --git a/src/lisp.h b/src/lisp.h
index 242156b..d44ab55 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -259,6 +259,11 @@ DEFINE_GDB_SYMBOL_BEGIN (EMACS_INT, VALMASK)
 # define VALMASK (USE_LSB_TAG ? - (1 << GCTYPEBITS) : VAL_MAX)
 DEFINE_GDB_SYMBOL_END (VALMASK)
 
+/* Ignore 'alignas' on compilers lacking it.  */
+#if !defined alignas && !defined __alignas_is_defined
+# define alignas(a)
+#endif
+
 /* Minimum alignment requirement for Lisp objects, imposed by the
    internal representation of tagged pointers.  It is 2**GCTYPEBITS if
    USE_LSB_TAG, 1 otherwise.  It must be a literal integer constant,