seperat link command for Winsock

[Davknapp]

Title Use seperate link command for WINSOCK_LIBRARY

Following up on issue cburstedde/libsc#206

Proposed changes: Use a seperate command instead of a generator expression. It prevents linker-errors with p4est if these expression fails to be properly generated.

[cburstedde]

Thanks again! I understand it's not originating from your code, but I'm wondering about an asymmetry with libsc: There we check for WIN32, here for HAVE_WINSOCK2_H. I would imagine this can be unified, with the goal to reduce the diff between the cmake environments of the two libraries as much as we possibly can.

[lukasdreyer]

@cburstedde Are you willing to merge this without the change? We have some users running into problems with it. I can change HAVE_WINSOCK2_H into WIN32 here, but I do not have any ideas of the technical implications.

[hannesbrandt]

I have been working on unifying the Winsock logic in p4est and libsc. It's still WIP, but most likely HAVE_WINSOCK2_H will stay in p4est as is and only libsc will be changed. So, from my perspective it would be fine to merge this PR and unify the Winsock logic with a separate PR in libsc later on.