update srt iOS build script

[spllr]

Updated the external dependencies scripts and add openssl and srt build triggers to the iOS Framework build.

These changes allow for installation with Carthage without manual installation steps.

• get latest iOS sdk version from xcrun
• run make after ./configure
• limit openssl archs to x86_64 i386 arm64 armv7s armv7
• run build scripts from script phase
• only run build script when libsrt-iOS.a is not present
• tweak README

[shogo4405]

👍

[spllr]

🤘🏼