Fix develop branch compile with OS X 10.8

This fixes the compile on the latest `develop` branch with OS X 10.8 by only setting `-std=c++11` when >- 10.9 is detected. Clang++ on OS X 10.8 by default links against libstdc++, and so the `-std=c++11` flag cannot be set unless `--stdlib=libc++` is also set otherwise the build will fail with the below error.

```sh
/usr/bin/clang++   -DOSRM_EXPORTS -Wall -Wno-unknown-pragmas -Wno-unneeded-internal-declaration -std=c++11 -O3 -DNDEBUG -fPIC -I/usr/local/include -I/usr/include/libxml2 -I/usr/local/include/luabind -I/usr/local/include/OSMPBF -I/Library/Frameworks/GDAL.framework/Headers    -o CMakeFiles/OSRM.dir/Library/OSRM.cpp.o -c /Users/dane/projects/Project-OSRM/Library/OSRM.cpp
In file included from /Users/dane/projects/Project-OSRM/Library/OSRM.cpp:28:
In file included from /Users/dane/projects/Project-OSRM/Library/OSRM.h:33:
In file included from /Users/dane/projects/Project-OSRM/Library/../Plugins/BasePlugin.h:31:
In file included from /Users/dane/projects/Project-OSRM/Library/../Plugins/../DataStructures/Coordinate.h:32:
In file included from /Users/dane/projects/Project-OSRM/Library/../Plugins/../DataStructures/../Util/StringUtil.h:35:
In file included from /usr/local/include/boost/spirit/include/karma.hpp:16:
In file included from /usr/local/include/boost/spirit/home/karma.hpp:13:
In file included from /usr/local/include/boost/spirit/home/karma/char.hpp:13:
In file included from /usr/local/include/boost/spirit/home/karma/char/char.hpp:15:
In file included from /usr/local/include/boost/spirit/home/support/string_traits.hpp:16:
In file included from /usr/local/include/boost/spirit/home/support/container.hpp:22:
In file included from /usr/local/include/boost/variant.hpp:17:
In file included from /usr/local/include/boost/variant/variant.hpp:31:
In file included from /usr/local/include/boost/variant/detail/initializer.hpp:23:
/usr/local/include/boost/variant/detail/move.hpp:100:12: error: no member named 'move' in namespace 'std'
using std::move;
      ~~~~~^
/usr/local/include/boost/variant/detail/move.hpp:139:36: error: no type named 'move' in namespace 'boost::detail::variant'
    T tmp( boost::detail::variant::move(lhs) );
           ~~~~~~~~~~~~~~~~~~~~~~~~^
/usr/local/include/boost/variant/detail/move.hpp:140:35: error: no member named 'move' in namespace 'boost::detail::variant'
    lhs = boost::detail::variant::move(rhs);
          ~~~~~~~~~~~~~~~~~~~~~~~~^
/usr/local/include/boost/variant/detail/move.hpp:141:35: error: no member named 'move' in namespace 'boost::detail::variant'
    rhs = boost::detail::variant::move(tmp);
          ~~~~~~~~~~~~~~~~~~~~~~~~^
```


Amends https://github.com/DennisOSRM/Project-OSRM/commit/4b3d634fe90cb2d72a772dada761749f4f5574d8#diff-af3b638bc2a3e6c650974192a53c7291. The following commit (https://github.com/DennisOSRM/Project-OSRM/commit/eaaf54d88640ddb5cabbca6e2e051de20bec1a26#diff-af3b638bc2a3e6c650974192a53c7291) is correct, but to fix the build the first needs removed.