Generate and install pkgconfig file

[tranzystorekk]

Adds a basic pkgconfig file generation. The main motivation for this is to enable linking a system-provided liboqs library in liboqs-rust.

I have done a simple manual test by creating a Void Linux package template for liboqs, installing it and checking that pkg-config detects its presence.

I am not proficient in pkgconfig, but I tried to follow the style found in some existing C libraries.

Closes #1434

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in oqs-provider, OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[tranzystorekk]

Some notes:

• I'm not sure how this could be easily tested; haven't taken the time yet to get familiar with the CI setup
• the Requires.private section probably should be set conditionally depending on whether openssl is enabled, but I wanted to get something out quickly

[thomwiggers]

liboqs often requires openssl, but this is optional

[tranzystorekk]

I'm thinking this would be best solved with an internal variable that is set to "openssl" if OQS_USE_OPENSSL is enabled. Is there some cmake variable naming convention in this project, e.g. _oqs_pkgconfig_requires_private?

[thomwiggers]

oops, I only intended to leave a comment, I don't have any experience with pkg-config so I can't otherwise meaningfully contribute.

[wucke13]

LGTM 😄

[wucke13]

@dstebila Now that liboqs has a pkgconfig file, can we create a minor release so that this becomes available in the distros?

[baentsch]

@wucke13 oqsprovider is also waiting for such release. But then again, there's only a single issue requiring resolution for 0.8.0 milestone... Further eyes welcome to help resolve this.