Export symbols for all compilers on Windows

[ChrisThrasher]

Description

Linking Catch2 v3.2.1 as a shared library when using Clang on Windows fails because symbols only get exported for MSVC (and MSVC-like compilers) but not for Clang. Removing this conditional means that Clang on Windows now gets this too. This should have no effect on compilers on non-Windows platforms. I believe we could move this out of the if(BUILD_SHARED_LIBS) block as well since it will just do nothing if you're building static libraries.

Here are two CI pipeline runs from SFML illustrating before and after applying this patch.

Before:
https://github.com/ChrisThrasher/SFML/actions/runs/3946193148

After
https://github.com/ChrisThrasher/SFML/actions/runs/3972002714

[codecov]

Codecov Report

Merging #2630 (baab9e8) into devel (2d3c971) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel    #2630   +/-   ##
=======================================
  Coverage   91.15%   91.15%           
=======================================
  Files         187      187           
  Lines        7691     7691           
=======================================
  Hits         7010     7010           
  Misses        681      681           

[horenmar]

Thanks.

I believe we could move this out of the if(BUILD_SHARED_LIBS) block as well since it will just do nothing if you're building static libraries.

After reading the docs, I agree. But since the conditional still has to be there (the behaviour of static libraries compiled with default visibility and hidden visibility is different when they are linked into dynamic library later), I think it is fine to leave it the way it is now.