Allow archs_for_command to take Pathnames. Fixes Homebrew/homebrew#1106.

Library/Homebrew/utils.rb

@@ -132,8 +132,9 @@ end
 
 # returns array of architectures suitable for -arch gcc flag
 def archs_for_command cmd
+  cmd = cmd.to_s # If we were passed a Pathname, turn it into a string.
   cmd = `/usr/bin/which #{cmd}` unless Pathname.new(cmd).absolute?
-    cmd.gsub! ' ', '\\ '
+  cmd.gsub! ' ', '\\ '  # Escape spaces in the filename.
 
   IO.popen("/usr/bin/file #{cmd}").readlines.inject(%w[]) do |archs, line|
     case line