CMake: Only use internal boost header (if available) for regression check

Fix a regression where a test for an icc regression in
check_01_cxx_features.cmake incorrectly used a generic <boost/signals2/...>
include. Such an include file doesn't have to be available at this early
point in the configuration run. User internal boost headers (if available)
instead - otherwise just test nothing.
Merge pull request #184 from tamiko/fix_cxx_test

CMake: Only use internal boost header (if available) for regression check