boost-libs fails to build on MacOS 10.11 -- how to fix?

[Greg Troxel <gdt%lexort.com@localhost>]

MacOS 10.11 is:

  Darwin-15.6.0-x86_64

with

  Apple LLVM version 8.0.0 (clang-800.0.42.1)
  Target: x86_64-apple-darwin15.6.0
  Thread model: posix
  InstalledDir: /Library/Developer/CommandLineTools/usr/bin

The problem is that libboost-fiber is created, but it is not expected:

  ERROR: The following files are in /n0/gdt/NetBSD-current/pkgsrc/devel/boost-libs/work/.destdir/opt/pkg but not in the PLIST:
  ERROR:         /n0/gdt/NetBSD-current/pkgsrc/devel/boost-libs/work/.destdir/opt/pkg/lib/libboost_fiber.a
  ERROR:         /n0/gdt/NetBSD-current/pkgsrc/devel/boost-libs/work/.destdir/opt/pkg/lib/libboost_fiber.dylib

So, I wonder if we should:

  1) let Darwin-15 have PLIST.fiber=yes

  2) add a .else so that on all systems where we do not set PLIST.fiber
  to yes, that we add an explicit --disable

  3) something else