Have Fgnutls_available_p return Qnil when GNUTLS is undefined

* src/gnutls.c (Fgnutls_available_p): Return Qnil when GNUTLS is
undefined to allow --with-gnutls=no builds to proceed.

src/gnutls.c

@@ -2435,6 +2435,8 @@ GnuTLS AEAD ciphers     : the list will contain `AEAD-ciphers'.  */)
 {
   Lisp_Object capabilities = Qnil;
 
+#ifdef HAVE_GNUTLS
+
 # ifdef HAVE_GNUTLS3
   capabilities = Fcons (intern("gnutls3"), capabilities);
 
@@ -2470,7 +2472,11 @@ GnuTLS AEAD ciphers     : the list will contain `AEAD-ciphers'.  */)
 
   return capabilities;
 
-#endif
+#endif /* WINDOWSNT */
+
+#else  /* !HAVE_GNUTLS */
+  return Qnil;
+#endif	/* HAVE_GNUTLS */
 }
 
 void