[Bug c++/121963] Make it easier to change the location of gcm.cache dir and document it

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121963

--- Comment #8 from Sam James <sjames at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #7)

(Seems done now.)