build: use automake subdir-objects

build: use automake subdir-objects

Automake 2.0 will enable subdir-objects by default; in preparation
for that change, automake 1.14 outputs LOADS of warnings:

daemon/Makefile.am:38: warning: source file '../src/remote/remote_protocol.c' is in a subdirectory,
daemon/Makefile.am:38: but option 'subdir-objects' is disabled
automake-1.14: warning: possible forward-incompatibility.
automake-1.14: At least a source file is in a subdirectory, but the 'subdir-objects'
automake-1.14: automake option hasn't been enabled.  For now, the corresponding output
automake-1.14: object file(s) will be placed in the top-level directory.  However,
automake-1.14: this behaviour will change in future Automake versions: they will
automake-1.14: unconditionally cause object files to be placed in the same subdirectory
automake-1.14: of the corresponding sources.
automake-1.14: You are advised to start using 'subdir-objects' option throughout your
automake-1.14: project, to avoid future incompatibilities.
daemon/Makefile.am:38: warning: source file '../src/remote/lxc_protocol.c' is in a subdirectory,
daemon/Makefile.am:38: but option 'subdir-objects' is disabled
...

As automake 1.9 also supported this option, and the previous patches
fixed up the code base to work with it, it is safe to now turn it on
unconditionally.

* configure.ac (AM_INIT_AUTOMAKE): Enable subdir-objects.
* .gitignore: Ignore .dirstamp directories.
* src/Makefile.am (PDWTAGS, *-protocol-struct): Adjust to
new subdir-object location of .lo files.

Signed-off-by: Eric Blake <eblake@redhat.com>