irrtoolset: modernize autotools deps

Formula/irrtoolset.rb

@@ -8,9 +8,9 @@ class Irrtoolset < Formula
   head do
     url 'svn://irrtoolset.isc.org/trunk'
 
-    depends_on :autoconf
-    depends_on :automake
-    depends_on :libtool
+    depends_on "autoconf" => :build
+    depends_on "automake" => :build
+    depends_on "libtool" => :build
   end
 
   depends_on 'pkg-config' => :build