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.