more issues with autoinclude detection

[Marc Slemko <ma...@znep.com>]

First, if you include some extra libraries that require something like
-lsocket, you have to explicitly add -lsocket to your EXTRA_LIBS line.
Otherwise, it won't find them for the includes tests so it won't find any
includes files so things will end up screwed later down the line.

We really need to check for something that is everywhere, then stop if
that isn't found so people have some clue that sort of thing is happening.

The other thing is that the test code generated uses "void main()".  That
isn't legal.