Bug#936017: mapnik: FTBFS with scons 3.1.1 (environment variables not used)

[Bas Couwenberg]

Source: mapnik
Version: 3.0.22+ds-2
Severity: serious
Tags: upstream
Justification: makes the package in question unusable or mostly so
Control: block 931949 by -1
Control: forwarded -1 https://github.com/mapnik/mapnik/issues/4081

Mapnik 3.0.22 FTBFS as part of the proj transition, seemingly due to
scons being updated to 3.1.1.

The environment variables set in debian/rules nor ones set in SConstruct
are actually used by the scons build.