Makefile.am cleanup

use "nobase_dist_" Automake prefix instead of adding a Makefile.am
file in each directory and linking them via SUBDIRS

See:
"Super" prefixes chapter in http://www.freesoftwaremagazine.com/articles/automatically_writing_makefiles_with_autotools
http://www.gnu.org/software/automake/manual/automake.html#index-nobase_005f
http://www.gnu.org/software/automake/manual/automake.html#Fine_002dgrained-Distribution-Control