Support for debug builds and/or configurable compiler flags

[SeanBryan51]

It would be useful if benchcab supported testing CABLE with debug builds and/or builds with configurable compiler flags so that we can catch any runtime exceptions or warnings that may be hidden in a release build (for example in #304 it would have been useful to catch the floating point overflow exception sooner).

This functionality comes by default with a spack based workflow (#286).