Implement a warn-on-undefined macro use flag in the macro engine, use
when parsing specs. We only cover %{...} macros as specs consist of numerous
directives starting with % that are not macros at all.

This uncovers a mountain of dirt, including in our own test-suite.

Use the new warning mechanism when expanding buildtree macros, such
as things only defined during spec parse (eg %{name}). That always
was a murky case that doesn't work in all scenarios, so a warning seems
appropriate. There's a slight side-effect in that the macros get now
expanded twice through rpmGetPath().