> I'm trying to get my package to depend on a specific upstream version of
> another package and ignore the Debian component of the version string.

For the Mono packages we use this snippet to get the upstream version in
debian/rules:
VERSION = $(shell dpkg-parsechangelog --show-field=Version)
UPVERSION = $(shell echo $(VERSION) | sed 's,-.*,,')
NEXT_UPVERSION = $(shell perl -e '$$_=pop; s/(\d+)$$/$$1+1/e; print' $(UPVERSION))

which is then passed to debian/control via:
dh_gencontrol -i -- -Vmono:upversion=$(UPVERSION) -Vmono:next-upversion=$(NEXT_UPVERSION)

then ${mono:upversion} and ${mono:next-upversion} can be used in debian/control