Adding variable to track stamped files in configure.ac

This is mimicing what was done in grinsfem/grins#127.
Moving .license_stamp target to main Makefile.am

Now using ANTIOCH_STAMPED_FILES variable
Enable subdir-objects option in Automake
Tabs, not spaces
Merge pull request #68 from pbauman/subdir_objects

Enable subdir-objects in Automake.