Add MacOS support

[thatcomputerguy0101]

This replaces malloc.h with stdlib.h, and adds discovery for MacOS libraries. The double *. in the library glob prevents matching the libopencv_java bundle dylib which cannot be loaded via fetchcontent.

[thatcomputerguy0101]

MacOS still needs added to the build workflow.

[mcm001]

Can/should we be building universal binaries?

[thatcomputerguy0101]

I added additional targets for macOS to the build action. I was going to try to get both targets to build on the Arm runner, but I was encountering a problem where the OpenCV arch got dropped if I tried to change the complier target.

[thatcomputerguy0101]

Can/should we be building universal binaries?

Maybe? I didn't try that yet.

[thatcomputerguy0101]

Building a universal binary seemed to work, which yields fewer artifacts to distribute.

[thatcomputerguy0101]

It seems like OpenBLAS may not be easy to compile a universal binary for. (I haven't eliminated the possibility yet though.)

[spacey-sooty]

Can we not link to Accelerate?

[thatcomputerguy0101]

Prior to macOS 26, Accelerate is 32-bit only. I only confirmed it passes the PV tests on macOS 26.

[Gold856]

To be honest, I think we use a 32 bit BLAS everywhere. I believe OpenBLAS is built with the LP64 API, so I think it's fine? And if it works with the ILP64 API, then I guess that's also fine?

[Gold856]

I'm gonna block only because I want a vanilla mrcal.