Add `subdir-objects` to configure.ac

Else throwing several errors as follows:

src/Makefile.am:2: warning: source file 'generated/ldb_actor.cpp' is in a subdirectory,
src/Makefile.am:2: but option 'subdir-objects' is disabled

automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.