rewrite fallthrough to HEIM_FALLTHROUGH to deal with new Apple SDKs

Apple clang version 14.0.0 (clang-1400.0.17.3.1) fails the build
because stds.h defines `fallthrough` as a macro which is then
expanded when base.h evaluates

  # if __has_attribute(fallthrough) && __clang_major__ >= 5

The macOS SDK defines `DISPATCH_FALLTHROUGH` as the macro instead
of `fallthrough`.

This change replaces the use of `fallthrough` in the tree with
`HEIM_FALLTHROUGH` and updates the declaration in configure logic
to define `HEIM_FALLTHROUGH` based upon existing definitions
(if any) of `fallthrough` or `DISPATCH_FALLTHROUGH`.

lib/roken/base32.c

@@ -100,10 +100,10 @@ rk_base32_encode(const void *data, int size, char **str, enum rk_base32_flags fl
 	p[6] = chars[(c & 0x0000000000000003e0ULL) >> 5];
 	p[7] = chars[(c & 0x00000000000000001fULL) >> 0];
         switch (i - size) {
-        case 4: p[2] = p[3] = '=';  fallthrough;
-        case 3: p[4] = '=';         fallthrough;
-        case 2: p[5] = p[6] = '=';  fallthrough;
-        case 1: p[7] = '=';         fallthrough;
+        case 4: p[2] = p[3] = '=';  HEIM_FALLTHROUGH;
+        case 3: p[4] = '=';         HEIM_FALLTHROUGH;
+        case 2: p[5] = p[6] = '=';  HEIM_FALLTHROUGH;
+        case 1: p[7] = '=';         HEIM_FALLTHROUGH;
         default:                    break;
         }
 	p += 8;

lib/roken/dirent-test.c

@@ -148,7 +148,7 @@ int teardown_test(void)
 
         strcmp(dirname + len + 1 - sizeof(TESTDIR)/sizeof(char), TESTDIR) == 0) {
 
-        fallthrough;
+        HEIM_FALLTHROUGH;
 
     } else {
         /* did we create the directory? */
@@ -162,7 +162,7 @@ int teardown_test(void)
                     fprintf(stderr, "Can't change to test directory. Aborting cleanup.\n");
                     return -1;
                 } else {
-                    fallthrough;
+                    HEIM_FALLTHROUGH;
                 }
             } else {
                 return -1;

lib/roken/fnmatch.c

@@ -129,7 +129,7 @@ rk_fnmatch(const char *pattern, const char *string, int flags)
 					--pattern;
 				}
 			}
-                        fallthrough;
+                        HEIM_FALLTHROUGH;
 		default:
 			if (c != *string++)
 				return (FNM_NOMATCH);

lib/roken/getuserinfo.c

@@ -136,7 +136,7 @@ roken_get_homedir(char *home, size_t homesz)
         }
         return home;
     }
-    fallthrough;
+    HEIM_FALLTHROUGH;
 #else
 #ifdef HAVE_GETPWNAM_R
     size_t buflen = 2048;

lib/roken/snprintf.c

@@ -515,7 +515,7 @@ xyzprintf (struct snprintf_state *state, const char *char_format, va_list ap)
 	    }
 	    case '\0' :
 		--format;
-                fallthrough;
+                HEIM_FALLTHROUGH;
 	    case '%' :
 		(*state->append_char)(state, c);
 		++len;

lib/roken/strftime.c

@@ -377,7 +377,7 @@ strftime (char *buf, size_t maxsize, const char *format,
 		break;
 	    case '\0' :
 		--format;
-                fallthrough;
+                HEIM_FALLTHROUGH;
 	    case '%' :
 		ret = snprintf (buf, maxsize - n,
 				"%%");

lib/roken/strptime.c

@@ -424,7 +424,7 @@ strptime (const char *buf, const char *format, struct tm *timeptr)
 		abort ();
 	    case '\0' :
 		--format;
-                fallthrough;
+                HEIM_FALLTHROUGH;
 	    case '%' :
 		if (*buf == '%')
 		    ++buf;