fix build on macOS
fix build on macOS

Without the `-stdlib=libc++` option the build fails with:

    [  5%] Building CXX object CMakeFiles/rapidcheck.dir/src/Check.cpp.o
    In file included from /Users/yairchu/dev/rapidcheck/src/Check.cpp:1:
    In file included from /Users/yairchu/dev/rapidcheck/include/rapidcheck/Check.h:21:
    In file included from /Users/yairchu/dev/rapidcheck/include/rapidcheck/Check.hpp:5:
    /Users/yairchu/dev/rapidcheck/include/rapidcheck/detail/Configuration.h:3:10: fatal error: 'cstdint' file not found

emil-e/rapidcheck,emil-e/rapidcheck,emil-e/rapidcheck