Changing priority for <compilerflag/>

I ran into an issue where I needed to change `-miphoneos-version-min` to `6.0`, and I couldn't find a way except by editing the "toolchain/iphoneos-toolchain.xml".

Would be nice if the `` would get appended at the end instead of the beginning so we can override some of those default values.

Or is there a way to overwrite those "toolchain/iphoneos-toolchain.xml"? Or maybe add a define for the min iphoneos that could be overrided?