How to change docdir in port Makefile?

[HU Dong]

Hi, all. I'm making a new port. It uses GNU configure to generate a
GNU Makefile. But it set DOCSDIR to
${PREFIX}/share/doc/${PORTNAME}-${PORTVERSION} instead
of ${PREFIX}/share/doc/${PORTNAME}:

***
docdir = $(prefix)/share/doc/$(PACKAGE)-$(VERSION)
***

I've tried post-patch, but it didn't help, cause post-patch phase is
earlier than the time when GNU Makefile is generated. So how to modify the
GNU Makefile to correct this problem?

B.R.
HU Dong