You need to check that all libraries is actually found by find_package
command. Easiest way
to do it is to add REQUIRED sub-option:
find_package(Boost 1.55 REQUIRED system serialization)
Works fine for me. Xcode 5.0.2, Boost 1.55, CMake 2.8.12.1, OS X 10.9. I'm using custom boost build (not system) with static libraries.
PS
IMHO there is no need to clear BOOST_ROOT
variable:
set(BOOST_ROOT CACHE PATH "Path to Boost library")
If boost is already found (by other parent project) you will do find-work twice, if project use custom boost location, you will rewrite it.