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

lib/http2.c

@@ -1505,7 +1505,7 @@ static int on_header(nghttp2_session *session, const nghttp2_frame *frame,
   }
 
   if(namelen == sizeof(HTTP_PSEUDO_STATUS) - 1 &&
-     memcmp(HTTP_PSEUDO_STATUS, name, namelen) == 0) {
+     !memcmp(HTTP_PSEUDO_STATUS, name, namelen)) {
     /* nghttp2 guarantees :status is received first and only once. */
     char buffer[32];
     size_t hlen;