feat: Support Swift's libBlocksRuntime

[ngrewe]

Some time ago, I added support in libobjc2 to using libBlocksRuntime as supplied by Swift's libdispatch instead of the embedded blocks runtime. This was done to alleviate the longstanding issue that both provided the same symbols which made properly packaging libobjc2 in distributions that also package libBlocksRuntime a bit of a hassle.

This PR changes gnustep-make to detect whether it needs to pull in libBlocksRuntime for blocks support. I also extracted the blocks feature checks into a separate m4 file and fixed a small typo in gs_objc_runtime.m4.

[triplef]

Thanks @ngrewe!

@hmelder with this merged can/should we switch to the libdispatch blocks runtime by removing this patch and building libdispatch before libobjc2?

[hmelder]

I would stick to the libobjc2 runtime and study the implementation in swift first.

[rfm]

I don't see any problems with this.