Prefer pkg-config while looking for BLAS

[marmistrz]

It turns out that BLAS is still not working on Arch Linux and FindBLAS.cmake fails to find libopenblas_64.so provided by openblas64 (note that Arch Linux does not have separate -dev packages).

This fixes it on Arch Linux. Even though the library libopenblas.so (32-bit) is detected, the performance difference is negligible on my system and it's better than nothing at all.

[bobqianic]

the performance difference is negligible on my system and it's better than nothing at all.

That's odd. I'd expect OpenBLAS to perform much better than ggml on a CPU. I'll review this PR soon. Can you provide more details about your hardware and system?

[marmistrz]

the performance difference is negligible on my system and it's better than nothing at all.

That's odd. I'd expect OpenBLAS to perform much better than ggml on a CPU. I'll review this PR soon. Can you provide more details about your hardware and system?

I mean the difference between openblas and openblas64, not openblas and ggml

[bobqianic]

I mean the difference between openblas and openblas64, not openblas and ggml

Ah, I see.

[marmistrz]

Btw., the reason why we can't find openblas64 with CMake without pkgconfig are these lines:
https://github.com/Kitware/CMake/blob/bc82d62b1bb484aabb1fdbb563c44f6e7cffb55f/Modules/FindBLAS.cmake#L748-L750

If the lines are changed to

  if(_blas_sizeof_integer EQUAL 8)
    string(APPEND _blas_openblas_lib "_64")
  endif()

then /usr/lib/libopenblas_64.so is correctly detected without having to use pkgconfig.