Warn if a keg-level "man" is found.

Homebrew expects manpages to be linked in shared/man/...
and not man/...

Library/Homebrew/install.rb

@@ -146,6 +146,14 @@ def install f
       end
     end
 
+    # Check for possibly misplaced folders
+    if (f.prefix+'man').exist?
+      opoo 'A top-level "man" folder was found.'
+      puts "Homebrew requires that man pages live under share."
+      puts 'This can often be fixed by passing "--mandir=#{man}" to configure,'
+      puts 'or by installing manually with "man1.install \'mymanpage.1\'".'
+    end
+
     # link from Cellar to Prefix
     begin
       Keg.new(f.prefix).link