Tabs and Makefile

The next (unreleased) version of Make is more strict about tabs too. Specifically this:

> Extra spaces are allowed and ignored at the beginning of the conditional directive line, but a tab is not allowed.

Currently tabs are allowed, but if you use `master` it will behave differently and give you an impossible to debug error (something like "file ended without endif").

In practice quite a lot of makefiles do indent conditionals with tabs.