Strip whitespace from OPTION_VALUE (fixes #635)

[dwuertz]

If key and value in OPTIONS lines are separated by more than a single space, the remaining whitespace is currently included in the OPTION_KEY, apparently causing the setting to be ignored (see #635), for some reason I don't fully understand. Stripping the excess whitespace fixes #635 and seems to be a reasonable thing to do in general.

[flagarde]

Maybe CPM should force -D := (https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-D) syntax