libBlocksRuntime: Mark public symbols with dllexport

[qmfrederik]

Functions which are marked with BLOCK_EXPORT are supposed to be externally visible, but miss the __attribute__((dllexport)) attribute. As a result, they are not exported from a .dll when compiled with clang.

Additionally, methods marked with __attribute__((dllexport)) cannot have the __attribute__((weak)) attribute applied to them, so make that conditional on being compiled for non-Windows platforms.

Together with #19767, this helps compiling Objective C code on MinGW using the clang compiler.

[Biswa96]

Thank you for the contribution. Just curious, should there be any condition for static vs. shared libraries?

[qmfrederik]

That's a good question. It doesn't look like the blocksruntime build setup is set up to really differentiate for this at build time -- it uses the same object files for creating the static and shared library:

libtool: link: clang -shared  .libs/libBlocksRuntime_la-runtime.o .libs/libBlocksRuntime_la-data.o    -g -O2 -fuse-ld=lld   -o .libs/libBlocksRuntime-0.dll -Wl,--enable-auto-image-base -Xlinker --out-implib -Xlinker .libs/libBlocksRuntime.dll.a
libtool: link: ar cr .libs/libBlocksRuntime.a  libBlocksRuntime_la-runtime.o libBlocksRuntime_la-data.o

Additionally, I don't really see a use case where you would want to use this as a static library. It's part of the (modern) Objective C runtime. Your application probably uses multiple shared libraries, all of which would reference the blocks runtime as a shared library.

It doesn't look like this package has been maintained very will since it was added 7 years ago (#3123). It also seems no other package depends on this.

A option could be to just stop publishing static versions of libblocksruntime?

@Adsun701 any thoughts?

[Adsun701]

@qmfrederik I think it would be a good idea to stop using static versions of libblocksruntime.

[qmfrederik]

@Biswa96 @Adsun701 I've removed the static build of libblocksruntime. Let me know if there's anything else you need!

[Adsun701]

@qmfrederik @Biswa96 I think that's good!