Building tests on Windows fails with `BUILD_SHARED_LIBS=ON`

[MiguelCompany]

Build fails with an unresolved external error:

default_allocator.obj : error LNK2001: unresolved external symbol "unsigned __int64 const foonathan::memory::virtual_memory_page_size" (?virtual_memory_page_size@memory@foonathan@@3_KB)

I've seen you are setting CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=ON here, but note the following sentence, extracted from the CMake docs:

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

It seems we need a __declspec(dllimport) here

[foonathan]

I'm not using windows and have never used DLL's, so I'd appreciate a PR.

[MiguelCompany]

An easy but API-breaking solution would be to convert that extern const into a method. That way, it will be automatically exported and imported.

Building the tests on the CI with BUILD_SHARED_LIBS on and off would be enough to check everything works ok.

[MiguelCompany]

@foonathan Please check #132 for a possible fix!