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

docs/examples/ghiper.c

@@ -399,7 +399,7 @@ int init_fifo(void)
   const char *fifo = "hiper.fifo";
   int socket;
 
-  if(lstat(fifo, &st) == 0) {
+  if(!lstat(fifo, &st)) {
     if((st.st_mode & S_IFMT) == S_IFREG) {
       errno = EEXIST;
       perror("lstat");