Add automake configuration option 'subdir-objects'

automake 1.14.1 complains:

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.
Makefile.am:128: warning: source file 'src/Basescape/BasescapeState.cpp' is in a subdirectory,
Makefile.am:128: but option 'subdir-objects' is disabled
(last two lines are repeated for every source file in subdirectories).

This commit adds the recommended option, and silences the warnings.