Coerce flags to an array in `remove_make_var!`

This is needed for Ruby 1.9.x compatibility, but we still don't support
running Homebrew with anything other than a system provided 1.8.[67].

Fixes Homebrew/homebrew#6546

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/extend/string.rb

@@ -22,6 +22,8 @@ module StringInreplaceExtension
 
   # Removes variable assignments completely.
   def remove_make_var! flags
+    # Next line is for Ruby 1.9.x compatibility
+    flags = [flags] unless flags.kind_of? Array
     flags.each do |flag|
       # Also remove trailing \n, if present.
       gsub! Regexp.new("^#{flag}[ \\t]*=(.*)$\n?"), ""