nvdaHelper sconstruct file: add a targetArchitectures commandline variable which...

nvdaHelper sconstruct file: add a targetArchitectures commandline variable which allows you to provide a comma-separated list of specific target architectures you want NVDAHelper built for. Example: scons targetArchitectures=x86,x86_64. By default all architectures will still be built like normal, but this provides an easy way of only building specific ones if needed. Also now detect any unknown commandline variables and report them and exit if any exist. Useful if you miss-spell a variable.