GRINS_STAMPED_FILES determined at configure time

This is an alternative to using a GNU make style
to get all the source file names to feed to the
header/license perl script.
Documenting alternative GNU Make style for grabbing files

This is in case the AC_SUBST version breaks and we need to do
something else. In theory, the AC_SUBST version is more portable
because it doesn't rely on GNU Make, but it is relying on some
autoconf foo that wouldn't surprise me if it broke. So, we'll
have this here to fall back on
Don't need those stamped files anymore
We don't need $(top_srcdir) in definition of _SORUCES anymore

This is because we've moved developing the list of source files to
feed the perl script header updater to configure.ac. This is
in preparation for turning on subdir-object
Turn on subdir_objects
Forgot to remove debugging echo
6 lines -> 1 line fix from @roystgnr
2 lines -> 1 line fix from @roystgnr
Merge pull request #127 from pbauman/subdir_objects

Subdir objects