Add warning about m4 macros.

Library/Homebrew/install.rb

@@ -182,6 +182,14 @@ def install f
       end
     end
 
+    # Check for m4 files
+    if Dir[f.share+"aclocal/*.m4"].length > 0
+      opoo 'm4 macros were installed to "share/aclocal".'
+      puts "Homebrew does not append \"#{HOMEBREW_PREFIX}/share/aclocal\""
+      puts "to \"/usr/share/aclocal/dirlist\". If an autoconf script you use"
+      puts "requires these m4 macros, you'll need to add this path manually."
+    end
+
     # link from Cellar to Prefix
     begin
       keg.link