Fix building as shared library with MSVC.

[dimztimz]

Fixes #2482.

For some more info see https://cmake.org/cmake/help/latest/prop_tgt/WINDOWS_EXPORT_ALL_SYMBOLS.html.

For global data symbols, __declspec(dllimport) must still be used when compiling against the code in the .dll.

See also this https://cmake.org/cmake/help/latest/prop_tgt/DEFINE_SYMBOL.html . That is where the define Catch2_EXPORTS comes from.

[codecov]

Codecov Report

Merging #2527 (63f198b) into devel (98d37da) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel    #2527   +/-   ##
=======================================
  Coverage   91.56%   91.56%           
=======================================
  Files         183      183           
  Lines        7560     7560           
=======================================
  Hits         6922     6922           
  Misses        638      638           

[horenmar]

Thanks. I am willing to merge this now, but I'd also like to see a PR that adds a test build for the dynamic library.