droption docs generation breaks cross-compilation

Our option parsing library (xref #1705) adopts a C++ template approach that has some nice qualities, but for automatically generating html documentation it requires a helper application to produce the docs at build time rather than config time. This does not work well with cross-compilation where build targets typically cannot be run locally.

For now we have docs disabled when targeting ARM so no live builds are broken but this will become an issues. There's no perfect solution: we may just have to disable docs when cross-compiling.