RFC: Bump minimum GNU Make version to 3.81

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 ;-)