fix: only check for the header unless Nokogiri provides LDFLAGS

in which case, then we verify that we can resolve libxml symbols.

Related to a41ab09 which checked symbol resolution on both Linux
and Windows; but it fails (and is unnecessary) on Linux, leading to
seeing this at installation:

> checking for xmlNewDoc() in libxml/tree.h... no

ext/nokogumbo/extconf.rb

@@ -75,7 +75,11 @@ def modern_nokogiri?
   if modern_nokogiri?
     append_cflags(Nokogiri::VERSION_INFO["nokogiri"]["cppflags"])
     append_ldflags(Nokogiri::VERSION_INFO["nokogiri"]["ldflags"]) # may be nil for nokogiri pre-1.11.2
-    have_libxml2 = have_func("xmlNewDoc", "libxml/tree.h")
+    have_libxml2 = if Nokogiri::VERSION_INFO["nokogiri"]["ldflags"].empty?
+                     have_header('libxml/tree.h')
+                   else
+                     have_func("xmlNewDoc", "libxml/tree.h")
+                   end
   end
 
   if !have_libxml2