Log Message:
boost-libs: Expect fiber to work on Darmin 15

fiber used to fail on Darwin 15, but as of 15.5 with Apple LLVM
>=8.0.0 (clang 8), it works.  Because we expect pkgsrc users to be up
to date with respect to minor versions of their (perhaps not
upgradable from) major version, change the regexp to expect fiber on
this platform.

ok minskin@ on tech-pkg