It appears that this support can be added simply by defining HAVE_ISC_READER in src/config.h. This line is already in the config.h for 2.31, but it's commented out. This option cannot be enabled with HAVE_BROKEN_RTC, but that is not enabled by default, either, and likely will never be, since it's for use with embedded systems without RTCs. I can provide a patch if you *really* want it, but the "fix" is pretty darn simple :)
There's an ebuils in bug 153917 which enables this stuff.
Added "isc" USE flag to the net-dns/dnsmasq-2.36 ebuild to build in ISC support.