tidy-up: drop stray comparisons with literal zero

Drop from:
- strcmp, strcmpi, strncmp, memcmp, lstat, getrlimit, setrlimit, fseek,
  fstat
- autotools detection snippets.
- smooth-gtk-thread: simplify `!var != 0` expression.

Closes #21947

src/tool_vms.c

@@ -59,7 +59,7 @@ int is_vms_shell(void)
   }
 
   /* Have to make sure some one did not set shell to DCL */
-  if(strcmp(shell, "DCL") == 0) {
+  if(!strcmp(shell, "DCL")) {
     vms_shell = 1;
     return 1;
   }