[Qgis-psc] Sponsored bug fixing, gdal 3 and QGIS 3.4

[Nyall Dawson]

Hi PSC,

Just looking for approval for me to spend some of the previously
agreed upon bug fixing hours for 3.8 on preparing the 3.4 branch for
compatibility with GDAL 3.0 and proj 6.

Currently, the branch will NOT compile and function on platforms which
have upgraded to GDAL 3.0/proj >= 6. Given that there's still a
lengthy time we need to support 3.4 for, I think we need to add this
compatibility or our LTR release will be broken on fast-moving
platforms.

I estimate 1 (at the worst 2) days would be required to do this, and
I'd love to do it now while the required changes are fresh in my mind
(due to ICSM's sponsored proj 6 work on 3.8)

For clarification: I do not intend to backport full proj 6 handling to
3.4 -- I only intend to fix the build and get the CRS db population
working again under GDAL 3.0. It will be a minimal set of changes in
order to fix the build only. Full proj 6 support is WAY to
risky/impossible to backport.

Nyall