libast build: fix circular dependency (re: 54a3e1c)

The generated depenency rules file for libast, arch/*/lib/mam/ast,
currently shows a circular dependency: ast_std.h → regex.h →
ast_wchar.h → stdio.h → ast_stdio.h → ast_std.h. As a result,
ast_stdio.h is not correctly depending on ast_std.h. The mkdeps
script indicates this with a comment under ast_stdio.h:
	note * FIXME: circular dependency: ast_std.h

We have to break this depenency loop somewhere. The best place to
break it is in ast_wchar.h, which is generated from features/wchar.
It has a '#include <stdio.h>', loading Sfio's stdio compatibility
interface as a hack to avoid the system's native wchar.h (on Linux
in particular) from pulling in native system stdio.h stuff. We
don't really need the dependency resolution for that #include.

src/cmd/INIT/mkdeps.sh:
- Add a feature to exclude individual #include directives from
  dependency resolution: when grepping for #include directives in
  the preinstalled AST headers, skip any line containing NODEP.

src/lib/libast/features/wchar:
- Add /* NODEP */ to the offending '#include <stdio.h>'.

Author: McDutchie

src/cmd/INIT/mkdeps.sh

@@ -1,7 +1,7 @@
 ########################################################################
 #                                                                      #
 #              This file is part of the ksh 93u+m package              #
-#             Copyright (c) 2024 Contributors to ksh 93u+m             #
+#          Copyright (c) 2024-2026 Contributors to ksh 93u+m           #
 #                      and is licensed under the                       #
 #                 Eclipse Public License, Version 2.0                  #
 #                                                                      #
@@ -20,6 +20,9 @@
 # dependency rules for a library. The 'bind' command in mamake(1) Mamfiles will
 # automatically include the generated dependencies in its current rule context.
 #
+# To exclude a #include from dependency resolution, add a comment with 'NODEP'.
+# This is (rarely) needed in cases of a circular dependency or optional header.
+#
 # Usage: mkdeps -lLIBRARYNAME [ -lDEPENDNECYNAME ... ] [ HEADER.h ... ]
 # The first -l option's argument is the short name of the library to be processed.
 # The second and further -l options indicate its library header dependencies.
@@ -56,7 +59,7 @@ error_out()
 grep_includes()
 {
 	spc=' 	'  # space followed by tab
-	sed -n "s|^[$spc]*#[$spc]*include[$spc]*<\([A-Za-z0-9_]*\)\.h>.*|\1|p" "$1"
+	sed -n "/NODEP/!s|^[$spc]*#[$spc]*include[$spc]*<\([A-Za-z0-9_]*\)\.h>.*|\1|p" "$1"
 }
 
 print_indent()

src/lib/libast/features/wchar

@@ -20,7 +20,7 @@ run{
 	cat <<!
 	#ifndef _SFSTDIO_H
 	#include <ast_common.h>
-	#include <stdio.h>
+	#include <stdio.h> /* NODEP */
 	#endif
 	!
 	case ${_hdr_wchar},${_nxt_wchar} in