Currently we document that GNU Make 3.80 is required. That was released in 2002. GNU Make 3.81 was released in April 2006. I would like to suggest that 20 years is enough time for 3.81 to be widespread.
Bumping the requirement to 3.81 would allow us to use the $(abspath ...) function, which we already started using in libstdc++, not realising that it relies on a newer version than our docs require. 3.81 also has $(lastword ...), $(realpath ...), $(or ...), and $(and ...). GNU Make 3.82 from 2010 adds the .ONESHELL feature but maybe 2010 is too modern for our tastes ;-)
