As per IEEE Std 1003.1-2017, Section 8.3, TERM and
SHELL vars says we are running GIMP on POSIX shell.
They are not set by either the deprecated Windows CMD
nor by Windows PowerShell since they are not POSIX.
POSIX shells are supported by us on Linux and macOS but
actually not on Windows, since they are non-native: they
rely on forking to Cygwin which have terrible performance
(##11385) and do other tricks to make deps believe they
are on Unix which breaks e.g. GObject Introspection (#11424).
Anyway, it is Unix emulation at its prime, tricky by nature.
Even so, does not hurt allow the output on these
unsupported shells on Windows so not disrupt workflows
of adventurous developers but they will be warned that
problems (like the ones above) can and indeed appear.
72cb583489