CMakeLists needs a fix for some hardcoded flags

[barracuda156]

Some of the hardcoded flags here are wrong: https://github.com/nmslib/hnswlib/blob/master/CMakeLists.txt

1. -march= is not supported on PowerPC, -mtune= is.

2. -lrt is a BSD library, it should not be used on macOS, AFAIK.

[yurymalkov]

Hi @barracuda156,
Can you please fix it wit a PR (and maybe test)?

[barracuda156]

@yurymalkov Will it run CI here? I will check on macOS locally, but I cannot check it on Linux or BSD.
#550