Re-allow `configure-option` in config file (re #2122)

A couple of settings were filtered out since
74cb2909 including
`configure-option`. However, it turns out you may want to
persist some options to `configure`, such as e.g.

  --configure-option=--build=x86_64-w64-mingw32

when working in a Cygwin host environment, but using a Mingw
toolchain (such as the one bundled with the Windows GHC bindist).

Approved by @dcoutts