Import exported.sh from cdecl99.

This script is required for the glconfig.mk snippet to actually
work, but it somehow got forgotten when that snippet was imported.

Bring in the script, and add M4 plumbing to support finding auxilliary
files in the dxcommon source directory.  The result should be totally
automatic, although this unfortunately means that users of fix-gnulib.pl
that don't need or want to use glconfig.mk are stuck with some extra
dependencies (including libtool) to generate the exported.sh script.

Currently, I can't think of a way to keep the automatic glconfig.mk
support (i.e., the only thing users really have to do is include the
snippet for it to work) without this spurious dependency.  So things
may need to change later, but let's just get the support in for now.