osx: switch of boost chrono to allow typegen-generated typekits using rt_string

Signed-off-by: Ruben Smits <ruben@intermodalics.eu>
Merge pull request #38 from smits/master

Fix visibility flag for clang and intel compilers
Merge pull request #41 from smits/osx-typegen-rt_string

osx: switch of boost chrono to allow typegen-generated typekits using rt_string