Working on windows builds (#7381)

Working on windows builds (#7381)

This makes a push to fix our windows nightly builds. It does a couple of things:
- remove `CVC5_WINDOWS_BUILD` and check `CMAKE_SYSTEM_NAME` instead
- use `CMAKE_SHARED_LIBRARY_SUFFIX` instead of `so`
- properly set `IMPORTED_IMPLIB` on imported shared libraries
- fix the GMP build, where the header is different for shared and static builds
- fix the find&sed patching for libpoly
- add `CVC5_EXPORT` to nested types in the API
- remove `CVC5_EXPORT` from enums
- add `-Dcvc5_obj_EXPORTS` to actually enable exports
- Fix how template function `safe_print` is exported.