Add some backwards-compatibility helpers for silent-rules.

The silent-rules support in Automake has changed over the ages.  Most
glaringly, the output alignment changed between 1.11 and 1.12.  Add a
new macro, DX_AUTOMAKE_COMPAT, which is intended to capture the
compatibility hacks needed to support multiple versions of Automake.
Currently, this means that it will define a variable to help print
silent-rule output that maintains alignment, and it will AC_SUBST
compatible versions, if necessary, of variables substituted in later
versions of Automake's silent-rule support.

Update glconfig.mk to make use of the new feature.