Dependencies: forwarding top-level CMAKE_CXX_FLAGS to each added external project

The ability to compile the Polycode dependencies against Clang standard library implementation was missing on OS X 10.8 / XCode 5 (where the default standard library is an old GNU version). 
see http://polycode.org/forum/viewtopic.php?f=4&t=869

This commit just forward CMAKE_CXX_FLAGS to each compiled dependency. It is then possible to set this variable to "-stdlib=libc++" to change the used std lib.
Default behavior does not change !