Ensure that NT2_INSTALL_SHARE_DIR is set when used in CMake

I ended up with:

Install the project...
-- Install configuration: ""
CMake Error at cmake_install.cmake:39 (FILE):
  file cannot create directory: /tools/is_supported.  Maybe need
  administrative privileges.
Fix compile error: a user provided default ctor is required here

The clang 3.2 generated error is:

nt2/modules/test/benchmark/src/priority.cpp:70:33: error:
          default initialization of an object of const type 'const
          nt2::details::priority_booster' requires a user-provided
          default constructor
  static priority_booster const priority_booster;
                                ˆ
OSX does not define "__unix" macro, uses __APPLE__ && __MACH__ instead

See: http://sourceforge.net/p/predef/wiki/OperatingSystems/#macos
Merge pull request #437 from joker-eph/OSX

Fix a few build issues on MacOS