png2ico: fix build with recent clang

This project seems dead, but several people reported this issue so let's
carry this tiny patch rather than remove it.

Fixes Homebrew/homebrew#23497.

Formula/png2ico.rb

@@ -7,6 +7,9 @@ class Png2ico < Formula
 
   depends_on :libpng
 
+  # Fix build with recent clang
+  def patches; DATA; end
+
   def install
     inreplace 'Makefile', 'g++', '$(CXX)'
     system "make", "CPPFLAGS=#{ENV.cxxflags} #{ENV.cppflags} #{ENV.ldflags}"
@@ -14,3 +17,17 @@ class Png2ico < Formula
     man1.install 'doc/png2ico.1'
   end
 end
+
+__END__
+diff --git a/png2ico.cpp b/png2ico.cpp
+index 8fb87e4..9dedb97 100644
+--- a/png2ico.cpp
++++ b/png2ico.cpp
+@@ -34,6 +34,7 @@ Notes about transparent and inverted pixels:
+ #include <cstdio>
+ #include <vector>
+ #include <climits>
++#include <cstdlib>
+ 
+ #if __GNUC__ > 2
+ #include <ext/hash_map>