Make CPACK Generator for Windows a cache variable
Currently the generator is set to `NSIS`, if windows is used with a normal variable.
We should use a cache variable instead, so the user is still able to change the generator per CLI of cpack. This could be neccessary, if one would have the 64bit version of `NSIS`, whose generator is `NSIS64`, or one would want a simple ZIP for distribution to people without admin access on their machines.
Change:
```cmake
set(CPACK_GENERATOR "NSIS")
```
To
```cmake
set(CPACK_GENERATOR "NSIS" CACHE STRING "Generator used by CPack")
```
(While we're at it, we could add a note, that all libraries for PeTrack need to be accessible in the path, i.e. petrack.exe should be startable with just a double click, for fixup_bundle and otools)
issue