Actually, there might not be. In MacPorts someone added patch for the version in configure.ac, but they simultaneously added a patch to Makefile.am for the version of the docs to be in an unversioned directory:

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

However I don't know if this was something that they intended to be incorporated upstream.

I checked with the author of the MacPorts patches: macports/macports-ports@8832f8a#r41406597
The patch to make the docs directory unversioned was for consistency with other projects in MacPorts. It is up to sparsehash whether they would like to adopt this change—it might be a good idea for them to, but if they can't/shouldn't, then MacPorts has no problem with continuing to patch it.

I use the C++11 version here myself as it's header only so no configure
https://github.com/sparsehash/sparsehash-c11

perhaps this is an option for the macports folks, as they likely wouldn't need to maintain it