Using pkg-config to find `libomp` on mac m1.

[Pradyot Ranjan]

On macOS, I have to use compiler options to find libomp, can they be
removed and we can use pkg-config from Homebrew that should be able to do
it automatically? The reason for this is because Apple Clang does not come
with the OpenMP specification – however, other compilers such as LLVM and
GCC do.

I tried doing `pkg-config --exists --print-errors libomp` but it throws
errors:

```
Package libomp was not found in the pkg-config search path.
Perhaps you should add the directory containing `libomp.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libomp' found
```
But with `clang -Xpreprocessor -fopenmp -I/opt/homebrew/opt/libomp/include
-o test test.c`
I'm able to compile successfully!
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