Add build options as configuration choices

[biddisco]

When builds are started, currently, we use a fixed set of options for each 'machine'. A new config file with a different machine name can be used to create a second set of options for a the same machine, but it would be better to allow many options/choices to be setup and allow pycicle to pick some either at random - or according to some other rule.

Options would generally fall into categories

• Compiler choice
• Compiler flags (e.g. c++11/14 or sanitizer flags)
• CMake options (WITH_XXX)
• Library or dependency versions (boost/hwloc/other)

[biddisco]

Closing this and using #13 instead