Fix C, Mutex imports in Poco 1.11.1

[aornugent]

In this Dockerfile I updated the FLINT dependencies from those given in the FLINT Dockerfiles:

FLINT base Dockerfile

Dependency	old version	new version
Ubuntu	18.04 (bionic)	20.04 (focal)
Poco	1.9.2	1.11.1
GDAL	2.4.1	3.4.1
Boost	1.72.0	1.78.0
fmt	6.1.2	8.1.1

Moving to Poco 1.9.2 this presented three minor bugs during FLINT compilation:

• The Poco Mutex headers were not being found for some reason, explicitly importing them in the writesystemconfig and writevariablegrid headers helped.
• I'm not quite sure why but we appear to have a C dependency that now needs to be explicit to Poco. This issue is being on the Poco project. Adding C as an enabled language in moja.modules.cbm/CMakeLists.txt resolved this.

@malfrancis - I'm in no great rush, but I'd love to know whether these minor changes are backwards compatible. Please let me know if you have time to trial this build.