Use FreeRADIUS's warning pragma controls to suppress Apple complaints.

Apple would really rather that you use their Shiny Happy Frameworks
rather than those crufty old cross-platform APIs.  We are a
cross-platform program, and will use platform-specific APIs only if
there's enough benefit to doing so - and, in this case, that means
"using the platform-specific APIs on OS X and the other APIs on other
platforms", so that's two code paths to maintain, so "enough benefit"
has to outweigh the issues with that.

Change-Id: I370ba469a6f5892143d72179d15c9fe22d664fdf
Reviewed-on: https://code.wireshark.org/review/3881
Reviewed-by: Guy Harris <guy@alum.mit.edu>