dev: quash gumbo compiler warnings 202412 (#3387)

**What problem is this PR intended to solve?**

I'm seeing some compiler warnings, so just doing some cleanup.

cc @stevecheckoway

CHANGELOG.md

@@ -4,7 +4,7 @@ Nokogiri follows [Semantic Versioning](https://semver.org/), please see the [REA
 
 ---
 
-## 1.18.0.rc1 / 2024-12-16
+## v1.18.0.rc1 / 2024-12-16
 
 ### Notable Changes
 

gumbo-parser/src/ascii.c

@@ -44,8 +44,8 @@ const unsigned char _gumbo_ascii_table[0x80] = {
 };
 
 // Table generation code.
-// clang -DGUMBO_GEN_TABLE=1 ascii.c && ./a.out && rm a.out
-#if GUMBO_GEN_TABLE
+// clang -DGUMBO_GEN_TABLE ascii.c && ./a.out && rm a.out
+#ifdef GUMBO_GEN_TABLE
 #include <stdio.h>
 
 int main() {

gumbo-parser/src/parser.c

@@ -318,7 +318,7 @@ static GumboNode* create_node(GumboNodeType type) {
   return node;
 }
 
-static GumboNode* new_document_node() {
+static GumboNode* new_document_node(void) {
   GumboNode* document_node = create_node(GUMBO_NODE_DOCUMENT);
   document_node->parse_flags = GUMBO_INSERTION_BY_PARSER;
   gumbo_vector_init(1, &document_node->v.document.children);
@@ -1997,7 +1997,7 @@ static void adjust_svg_tag(GumboToken* token) {
   assert(token->type == GUMBO_TOKEN_START_TAG);
   if (token->v.start_tag.tag == GUMBO_TAG_FOREIGNOBJECT) {
     assert(token->v.start_tag.name == NULL);
-    token->v.start_tag.name = "foreignObject";
+    token->v.start_tag.name = (char *)"foreignObject";
   } else if (token->v.start_tag.tag == GUMBO_TAG_UNKNOWN) {
     assert(token->v.start_tag.name);
     const StringReplacement *replacement = gumbo_get_svg_tag_replacement(