CMakeFindBinUtils sets CMAKE_RANLIB to `:` if it is not available in
order to get a no-op.  This does not work on a Windows host build
environment that runs commands in `cmd` instead of `sh`.  Teach the
Ninja and Makefile generators to simply skip the command if it is `:`.

This this was already done by the Makefile generator since commit
v2.6.0~3161 (BUG: Do not write link script lines that use the ':',
2006-06-18), but only when using a link script.

Reported-by: Michael Jäntsch <Michael.Jaentsch@gmx.de>