bpo-35569: Expose RFC 3542 socket options on macOS

From macOS SDK netinet6/in6.h:
RFC 3542 define the following socket options in a manner incompatible
with RFC 2292:
  IPV6_PKTINFO
  IPV6_HOPLIMIT
  IPV6_NEXTHOP
  IPV6_HOPOPTS
  IPV6_DSTOPTS
  IPV6_RTHDR

To use the new IPv6 Sockets options introduced by RFC 3542
the constant __APPLE_USE_RFC_3542 must be defined before
including <netinet/in.h>

[...]

Note that eventually RFC 3542 is going to be the
default and RFC 2292 will be obsolete.

Author: Erlend E. Aasland

Misc/NEWS.d/next/macOS/2020-04-15-00-02-47.bpo-35569.02_1MV.rst

@@ -0,0 +1 @@
+Expose RFC 3542 IPv6 socket options.

configure

@@ -7421,6 +7421,9 @@ $as_echo "$ac_cv_enable_visibility" >&6; }
 	    ;;
 
     Darwin*)
+        # Issue #35569: Expose the RFC 3542 socket options.
+        CFLAGS="-D__APPLE_USE_RFC_3542 ${CFLAGS}"
+
         # -Wno-long-double, -no-cpp-precomp, and -mno-fused-madd
         # used to be here, but non-Apple gcc doesn't accept them.
         if test "${CC}" = gcc

configure.ac

@@ -1844,6 +1844,9 @@ yes)
 	    ;;
 
     Darwin*)
+        # Issue #35569: Expose the RFC 3542 socket options.
+        CFLAGS="-D__APPLE_USE_RFC_3542 ${CFLAGS}"
+
         # -Wno-long-double, -no-cpp-precomp, and -mno-fused-madd
         # used to be here, but non-Apple gcc doesn't accept them.
         if test "${CC}" = gcc