Hopefully good fix for va_listval build fails (re: 41ed8047, etc.)

The last commit still failed to build on macOS M1. That va_listval
macro keeps causing trouble. It's an AST thing that is defined in
src/lib/libast/features/common. That looks like some incredibly
opaque attempt to make it compatible with everything, and clearly
it no longer works properly on all systems. I don't dare touch it,
though. That code looks like any minimal change will probably break
the build on some system or other.

src/lib/libast/features/hack:
- Add feature test to check if that macro needs (0) no workaround,
  or (1) the workaround from the 93v- beta, or (2) the FreeBSD one.
  Whichever version compiles first, it will use. If none does, this
  test will not output a value, which will be treated as 0.

src/lib/libast/hash/hashalloc.c,
src/lib/libast/string/tokscan.c:
- Update to use the result of the hack feature test.

src/lib/libast/Mamfile:
- Update for new #include dependencies.

src/lib/libast/Mamfile

@@ -523,6 +523,13 @@ make install
 			done fastfind.o generated
 			make hashalloc.o
 				make hash/hashalloc.c
+					make FEATURE/hack implicit
+						meta FEATURE/hack features/%>FEATURE/% features/hack hack
+						make features/hack
+							prev FEATURE/common implicit
+						done features/hack
+						exec - iffe ${IFFEFLAGS} -v -X ast -X std -c "${CC} ${mam_cc_FLAGS} ${KSH_RELFLAGS} ${CCFLAGS} ${LDFLAGS}" run features/hack
+					done FEATURE/hack generated
 					make hash/hashlib.h implicit
 						make include/hash.h implicit
 							make include/hashpart.h implicit
@@ -909,6 +916,7 @@ make install
 			done tokline.o generated
 			make tokscan.o
 				make string/tokscan.c
+					prev FEATURE/hack implicit
 					prev include/tok.h implicit
 					prev include/ast.h implicit
 				done string/tokscan.c
@@ -3118,12 +3126,7 @@ make install
 						make locale_attr.h implicit
 						done locale_attr.h dontcare virtual
 					done comp/gross_sgi.h dontcare
-					make FEATURE/hack implicit
-						meta FEATURE/hack features/%>FEATURE/% features/hack hack
-						make features/hack
-						done features/hack
-						exec - iffe ${IFFEFLAGS} -v -X ast -X std -c "${CC} ${mam_cc_FLAGS} ${KSH_RELFLAGS} ${CCFLAGS} ${LDFLAGS}" run features/hack
-					done FEATURE/hack generated
+					prev FEATURE/hack implicit
 					prev include/ls.h implicit
 					prev include/ast.h implicit
 				done comp/gross.c

src/lib/libast/features/hack

@@ -1 +1,51 @@
 hdr	locale_attr
+
+tst	- -DVAL=0 - -DVAL=1 - -DVAL=2 note{ probing need for va_listval() workaround }end output{
+	/*
+	 * This workaround test is needed until libast's va_listval macro from
+	 * features/common can be made to work properly on all systems (if ever).
+	 * It is used in hash/hashalloc.c and string/tokscan.c.
+	 */
+	#include "FEATURE/common"
+	#if _STD_ && _hdr_stdarg
+	#include <stdarg.h>
+	#else
+	#include <varargs.h>
+	#endif
+	void foo(int bar, ...)
+	{
+		va_list ap;
+		va_start(ap, bar);
+		if (bar==1)
+		{
+	#if VAL == 0
+			/*
+			 * Normal version. On some systems, it won't compile.
+			 */
+			va_copy(ap, va_listval(va_arg(ap, va_listarg)));
+	#elif VAL == 1
+			/*
+			 * This is the workaround from the ksh 93v- beta distribution.
+			 */
+			va_list np;
+			np = va_listval(va_arg(ap, va_listarg));
+			va_copy(ap, np);
+	#elif VAL == 2
+			/*
+			 * This is the workaround from the FreeBSD port.
+			 * https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255308
+			 */
+			va_listarg np;
+			np = va_listval(va_arg(ap, va_listarg));
+			va_copy(ap, np);
+	#endif
+		}
+		va_end(ap);
+	}
+	int main()
+	{
+		foo(0,"one",2);
+		printf("#define _need_va_listval_workaround %d\n",VAL);
+		return 0;
+	}
+}end

src/lib/libast/hash/hashalloc.c

@@ -31,6 +31,7 @@
 static const char id_hash[] = "\n@(#)$Id: hash (AT&T Research) 1996-08-11 $\0\n";
 
 #include "hashlib.h"
+#include "FEATURE/hack"
 
 Hash_info_t	hash_info = { 0 };
 
@@ -152,10 +153,13 @@ hashalloc(Hash_table_t* ref, ...)
 				va_copy(*vp, ap);
 				vp++;
 			}
-#if __clang__ && __SIZEOF_POINTER__ == 4 && !__APPLE__
+#if _need_va_listval_workaround
 			{
+#   if _need_va_listval_workaround == 2
+				va_listarg	np;
+#   else
 				va_list		np;
-
+#   endif
 				np = va_listval(va_arg(ap, va_listarg));
 				va_copy(ap, np);
 			}

src/lib/libast/string/tokscan.c

@@ -64,6 +64,7 @@
 
 #include <ast.h>
 #include <tok.h>
+#include "FEATURE/hack"
 
 static char	empty[1];
 
@@ -234,10 +235,13 @@ tokscan(register char* s, char** nxt, const char* fmt, ...)
 			prv_f = f;
 			f = va_arg(ap, char*);
 			va_copy(prv_ap, ap);
-#if __clang__ && __SIZEOF_POINTER__ == 4 && !__APPLE__
+#if _need_va_listval_workaround
 			{
+#   if _need_va_listval_workaround == 2
+				va_listarg	np;
+#   else
 				va_list		np;
-
+#   endif
 				np = va_listval(va_arg(ap, va_listarg));
 				va_copy(ap, np);
 			}