devel/glibmm: update to 2.66.2

Converted to meson.
PORTSCOUT limited to the 2.66 series as 2.68 and later are incompatible
with current consumers; the later versions are mainly for GTK 4
consumers that AFAICT do not exist yet.

MFH: 2 weeks
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D33938