cmake: fixing Clang on OSX

compilers and their flags must be passed on to 3rdparty libs

libc++ must be enabled on OSX systems (couldn't get Clang with GNU
libstdc++ to work)

fixes #9

Signed-off-by: Torbjörn Klatt <t.klatt@fz-juelich.de>
Merge pull request #12 from torbjoernk/feature/fix-osx-clang

cmake: fixing Clang on OSX