[m-rev.] for post-commit review: fix unneeded recompilations

[Zoltan Somogyi]

This has been bugging me for a while.

Note that I am using autoconf 2.71, which is
one version behind the latest. If you use the latest,
and this diff does not fix the issue for you, please tell me.

For review by anyone.

Zoltan.
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