Update GCC_DEFAULT from 9 to 10.
A few (mostly Fortran-based) ports need to be patched because GCC 10
is stricter. A handful of ports need to be restricted to GCC 9 because
they fail with 10.
5 ports still fail with GCC 10. However, seeing as this work has been
ongoing since 2020-05-24, it is simply time to make the commit and
notify the affected maintainers.
While here, pet portlint (Makevar order).
Submitted by: gerald
Approved by: antoine (after many, many, -exp runs)