Replace `CPL_DLL` definition with `CTB_DLL`

Using `CPL_DLL` requires changing GDAL's `cpl_port.h` file which is a hack too far on many systems so the change is instead encapsulated in `CTB_DLL`.  This also re-enables CMake directives that are appropriate for toolchains other than Visual Studio.

@Jule- would you be able to test these changes work for you with Visual Studio?  It may be that the CMake conditionals using the `MSVC` variable may need changing.  If so [this page](http://www.cmake.org/Wiki/CMake_Useful_Variables) may be useful.