Allow windows linking of test programs

[matlimatli]

The kat_kem and kat_sig programs could not be linked when building natively on windows. This was caused by multiple definitions of symbols. By using the /FORCE:MULTIPLE compiler option, this is allowed, similar in spirit to what was already used for cross-compiling to Windows.

Fixes #1749

• [No] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [No] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[baentsch]

Excellent! Thanks very much for this contribution @matlimatli !

Note to self and @smcmahonibm: Let's consider adding a CI test for this config if this is more widely used.

[SWilson4]

Thank you for the contribution!

[vsoftco]

@baentsch @SWilson4 @matlimatli I saw this a bit late.. the easiest (imo) fix is to use the CMake flag -D CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=TRUE when compiling for Windows, it takes care of the multiple defined symbols at linking. I use it on my side on a Windows VBox and works well.

[baentsch]

Thanks for the suggestion, @vsoftco . I do like the fix in this PR, though, as it doesn't depend on the setting of a cmake flag: Do you also see it like that or is there something "CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS" does "better"?

[vsoftco]

I don't think there's anything that CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=TRUE does better, I just pointed out that CMake now has the option (since I bumped into the exact same issue a while ago). The only think CMAKE may (although I'not sure) do better is ensuring compatibility across different compilers (say that some other Win compiler comes along, not MSVC). But again, totally fine with this PR as long as it fixes it and works.

[matlimatli]

@vsoftco I didn't know about that option. Thanks for pointing it out, it might come in handy in some situations!

[baentsch]

Worthwhile a documentation PR, e.g., here?