build: use abort to pacify compiler errors

clang, with -DNDEBUG and -Werror fails on some functions that might
lack a return.  This is because aver is just another assert, discarded
with -DNDEBUG.  So use abort.

* src/muscle-tab.c, src/scan-skel.l: here.

src/muscle-tab.c

@@ -37,7 +37,7 @@ muscle_kind_new (char const *k)
     return muscle_keyword;
   else if (STREQ (k, "string"))
     return muscle_string;
-  aver (0);
+  abort ();
 }
 
 char const *
@@ -49,7 +49,7 @@ muscle_kind_string (muscle_kind k)
     case muscle_keyword: return "keyword";
     case muscle_string:  return "string";
     }
-  aver (0);
+  abort ();
 }
 
 

src/scan-skel.l

@@ -192,7 +192,7 @@ flag (const char *arg)
   else if (STREQ (arg, "warn"))
     return Wother;
   else
-    aver (false);
+    abort ();
 }
 
 static void