Unbreak CMake build on OpenBSD

[klemensn]

• CMake: Limit procfs detection to Linux
• CMake: Use correct expression syntax

[ofalk]

Yet another good catch @klemensn ! I'll quickly pull this one as well.

[klemensn]

@ofalk by squashing my two commits and using this pull request's summary as commit message, you lost the original commit messages with actual information in the git history, i.e. one can now only see them through GitHub or if they ever fetched the commits from my tree.

Not super important for these fixes, but please consider avoiding squash merges -- or at least retain original messages.

[ofalk]

Got it @klemensn ! Sorry for that, I'll take care about it next time!