Better support for external boost (especially on windows)

Projects/boost.use.system.cmake

+set(BOOST_INCLUDEDIR "${BOOST_INCLUDEDIR}" CACHE PATH "Location of boost header files" FORCE)
+set(BOOST_LIBRARYDIR "${BOOST_LIBRARYDIR}" CACHE PATH "Location of boost library files" FORCE)
+
+set(Boost_NO_BOOST_CMAKE ON)
+find_package(Boost REQUIRED)
+
+# This is a hack.  If boost was found by some other mechanism then the
+# assumptions here would fail.
+add_extra_cmake_args(
+  -DBOOST_INCLUDEDIR:PATH=${BOOST_INCLUDEDIR}
+  -DBOOST_LIBRARYDIR:PATH=${BOOST_LIBRARYDIR}
+)