PATCH: Allow distdir to be overridden on make command line

Not just a one line patch, a one character patch.

Use ?= instead of = in distdir assignment, so it can be overridden on
the command line when building dist tarballs with patches.

Yes, you can just modify GNUMakefile.in, but that's extra noise in a
diff, adds merge conflicts, etc.

Please apply. Surely this is harmless?

--
 Craig Ringer                   http://www.2ndQuadrant.com/
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