The SSL*ALPNProtocols were introduced in recent OS version, and dynamic
lookup (using ld64 option -Wl,-U) was used to make it work during
link-time and potentially not have the function available.

However, -Wl,-U is not compatible with bitcode support, whereas
availability targeting is, and availability targeting provides more
information about when it's not supported.

MacOSX 10.13.4 is the minimal version target in cURL too. The
SSL*ALPNProtocols functions are documented as supported since 10.13 but
they were not included in the initial 10.13 releases.