DX_AUTOMAKE_COMPAT: Fix install-sh with newer Automake.

Current versions of Automake's install-sh depends on substitutions like
${var:-foo}, 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 install-sh starts with #!/bin/sh so when it is executed
directly, it is run with /bin/sh instead of the shell we want.

We can work around the problem by patching up the MKDIR_P and INSTALL
assignments at configure time to include the known-good shell, which
seems to work well.