make pkg-config a hard requirement for Android builds, since lzma requires it

from https://github.com/torproject/tor/pull/1408#discussion_r339033628

I can see that this lzma issue really concerns you. But I'm struggling to understand what you need us to change. Can you open another ticket with your request? I'm a bit confused by all the different topics in this one comment thread.

This is what I'm talking about:

We couldn't change it to an error, because that would break existing build scripts. https://trac.torproject.org/projects/tor/ticket/32191?replyto=21#comment:5

For Android builds with the current setup + https://github.com/torproject/tor/pull/1408, pkg-config is not optional, lzma will not be included if pkg-config is not there. I propose adding a hard error to the if test "x$enable_android" = "xyes"; then block if pkg-config is not present. That should be easy and work.

IMHO pkg-config is probably not needed at all for Android builds if legacy/trac#31882 (moved) and legacy/trac#32191 (moved) are included. But that's a bigger project. I don't think I'd be able to get that code past the review process without it spelled out here first.