hash.c, symbol.c, string.c, mruby-string-ext: undef lesser macro

Add #undef lesser after last usage to prevent macro redefinition
warnings when files are amalgamated into a single translation unit.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-string-ext/src/string.c

@@ -1226,6 +1226,7 @@ str_casecmp(mrb_state *mrb, mrb_value self)
   if (len1 > len2)  return mrb_fixnum_value(1);
   return mrb_fixnum_value(-1);
 }
+#undef lesser
 
 /*
  * call-seq: