Use subdir-objects for automake 1.14 or newer

Use subdir-objects for automake 1.14 or newer

This option is going to be mandatory starting with automake 2.0.
LyX is able to use this since version 2.1, but it turns out that it is
badly implemented in versions of automake older than 1.14.

We rely on some (undocumented) symbol to detect automake 1.14 and
use the subdirs-objects option in this case.

For more details, see:
https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg181023.html

Making everything work with autotools required making a copy of two
files from support/tests. It seems that we should not point to source files
that are handled by another Makefile.am.