Fix CI inconsistencies and missing DLLs

[mrikola]

• Use SDL 2.0.10 and 32-bit CMake in win32 build
• Move 32bit CMakeLists.txt to project root, use it for win32 builds
• Include libwinpthreads-1.dll in both Appveyor builds as they use POSIX MinGW builds

Due to the library version upgrade, there should be an Appveyor cache purge.

[mrikola]

Reminder on how to purge the cache:

1. Find API key from https://ci.appveyor.com/api-keys
2. DELETE https://ci.appveyor.com/api/projects/Interrupt/systemshock/buildcache with the API key as Bearer token

[Interrupt]

Purged the AppVeyor cache after the merge.