xplat: support build with Ninja
xplat: support build with Ninja

Add build.sh `-n, --ninja` option to build with Ninja instead of make.
(Ninja uses multicore build by default and doesn't mix output from
different cores.)

mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore