Allow jobs to pass additional compiler flags

Jobs can pass these additional flags in the variables DUNECI_CFLAGS and DUNECI_CXXFLAGS. This is similar to what is already possible with DUNECI_CMAKE_FLAGS, except that that variable cannot be used to pass additional compiler flags.

This is meant for flags that select particular sub-achitectures such as -march. Jobs still need to make sure to select a suitable runner actually supporting those architectures via runner tags.

Addresses: core/dune-common#165 (closed)

Note: I haven't tested this. If you want me to, I can probably set up a dune-common branch that copies the updated dune.opts over the one provided by the image...

merged

mentioned in commit 90b8c86d

mentioned in issue core/dune-common#165 (closed)