Compatibility updates to Makefile.am

Resolve the following issue during ./autogen.sh:

Makefile.am:4: warning: source file 'src/ta_ema.c' is in a subdirectory,
Makefile.am:4: 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.