CMake: Fix Build & Install Option Names (openPMD#1326)

* CMake: Fix Build & Install Option Names

We should not overwrite `CMAKE_` variables but instead set our
own. This corrects this.

This also fixes that the cmake install path was unset in
superbuilds failing the configure steps unless set by
the calling project did set `CMAKE_INSTALL_CMAKEDIR`:
```
CMake Error at openpmd-src/CMakeLists.txt:1080 (install):
  install EXPORT given no DESTINATION!

CMake Error at openpmd-src/CMakeLists.txt:1085 (install):
  install FILES given no DESTINATION!
```

* Win: LIBDIR changed from `Lib` to `lib`

* Examples & CLI: Also Set Lib Output