[patch][core] Use AS_AC_EXPAND instead of trying to make our own

[Havoc Pennington]

Simon McVittie wrote:
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> On a current Debian unstable system, ${datadir} is defined in terms of
> ${datarootdir}, so EXPANDED_DATADIR needs to be expanded recursively.
> Rather than fixing configure.in to do this, I grabbed the AS_AC_EXPAND
> macro from autostars.sf.net, which seems to be commonly used.
> 

Looks like a nice cleanup.

Havoc