Darwin (Mac OS & iOS) native SSL backend

[ming]

This ticket will add support of Darwin native SSL using the Security framework as the SSL backend for Mac OS and iOS.

[ming]

In 5957:

Fixed #2185: Darwin (Mac OS & iOS) native SSL backend

[ming]

In 5958:

Re #2185: Add Security framework to ipjsua sample app

[ming]

In 5959:

Re #2185: Darwin SSL backend

• Security framework availability will be automatically detected for Mac OS and iOS, and if found, Darwin SSL will be the default SSL backend.
• OpenSSL availability will still be checked regardless of the above result, as it may be needed for DTLS.
• Add configure option --disable-darwin-ssl to disable the automatic detection

[ming]

In 6018:

Re #2185: Modified the autodetection in configure script to try to compile a sample code instead.