When including an -internal.h header outside the main tree, do so early

Some of our unit tests and sample code need functions and structures
defined in an -internal.h header.  But that can freak out OpenSolaris,
where stdio.h wants to define _FILE_OFFSET_BITS unless it's already
defined, and then evconfig-internal.h defines it.  Regular users
should never ever use our -internal.h headers, so the solution is
to make sure that if we're going to use them ourselves, we do so
before system headers.