Pass arguments to tweak to build

Our build machine has several Xcode versions installed in /Application/Xcode-x.y.z.app
In order to build OpenSSL for a various set of iPhoneOS SDKs, I tweaked the script to support arguments.

Usage :
./build-libssl.sh [version] [Xcode path]

Example :
./build-libssl.sh 1.0.1h /Applications/Xcode-6.0.1.app

By default Xcode is in /Applications/Xcode.app :
./build-libssl.sh 1.0.1h /Applications/Xcode.app
is equivalent to
./build-libssl.sh 1.0.1h