DX_AUTOMAKE_COMPAT: Fix compile with newer Automake.

Current versions of Automake's compile script depend on shell functions
which are not supported by ULTRIX 4.5 /bin/sh.

Now ksh does support these and configure does find and use this shell
on ULTRIX, but compile starts with #!/bin/sh so when it is executed
directly, it is run with /bin/sh instead of the shell we want.

This is a bit tricky to work around since Automake updates $CC (if
needed) to refer to the compile script, which is used throughout
the configure run.  So we can't just patch it up whenever, it has
to be done early on.  AC_BEFORE can be used to at least warn if the
patches are ineffective.