Visual Studio compilation with CMake

Following our previous discussions (issue #6), I investigate in order to make some changes to CMake rules in order to make it work on Windows with Visual Studio.

The only prerequisite is to modify `cpl_port.h` in your GDAL include directory in order to define `CPL_DLL` declaration:
```c++
#ifndef CPL_DLL
#if defined(_MSC_VER) && !defined(CPL_DISABLE_DLL)
#  define CPL_DLL     __declspec(dllexport)
#else
#  if defined(USE_GCC_VISIBILITY_FLAG)
#    define CPL_DLL     __attribute__ ((visibility("default")))
#  else
#    if defined(_MSC_VER)
#      define CPL_DLL     __declspec(dllimport)
#    else
#      define CPL_DLL
#    endif
#  endif
#endif
#endif
```

If my modifications don't break linux compilation may be you can merge my changes.

HTH