问题描述:
因为windows XP和windows 7的program files不同(有program files(x86)),所以需要动态根据系统的位数设置默认安装目录
References:
http://*.com/questions/1502542/nsis-installdir-in-vista-xp
http://*.com/questions/11126629/use-one-nsis-installer-to-install-32-bit-binaries-on-32-bit-os-and-64-bit-binari/11127544#11127544