EigenTesting.cmake: Quote passed to separate_arguments

Per cmake documentation:

The entire command line must be passed as one string in the argument .

This is not currently done when setting compile_options, which can lead to build failures when multiple options are specified:

separate_arguments(compile_options NATIVE_COMMAND ${ARGV2})

should be:

separate_arguments(compile_options NATIVE_COMMAND "${ARGV2}")

See also: !1740 (merged). (I think this would have also fixed this one.)