Boost/Fuego build scripts now use same default base SDK + deployment target (#10)

new defaults
- base SDK = latest
- deployment target = 5.0

the defaults can be overridden with these envvars:
- IPHONEOS_BASESDK_VERSION
- IPHONEOS_DEPLOYMENT_TARGET
- IPHONE_SIMULATOR_BASESDK_VERSION
- IPHONE_SIMULATOR_DEPLOYMENT_TARGET