Eclipse: add -j only on UNIX hosts

The previous version added -jN to the make invokation if UNIX
was set, but this was true also when cross compiling.
What we are actually interested in is CMAKE_HOST_UNIX, i.e.
whether the build host is UNIX.
Or we should check more exactly what CMAKE_MAKE_PROGRAM is.

Alex
Merge topic 'EclipseParallelMakeOnlyOnUnixHosts' into next

83751af Eclipse: add -j only on UNIX hosts