Fix libdir value in liboqs.pc

[vt-alt]

Libdir is not always '/usr/lib' and its value is different between distributions and architectures. Set it from CMAKE_INSTALL_LIBDIR. Also, set includedir in a similar way.

Currently, this incorrect setting did not produce incorrect output by pkg-config --libs because pkg-config strips 'system library paths' form the output. But in non-standard build environments this may cause incorrect linking.

Issue: #1495

• 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 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.)

[vt-alt]

I tested on ALT and Debian and new values appear to me correct.
Debian builds with cmake setting -DCMAKE_INSTALL_LIBDIR=lib/x86_64-linux-gnu and on ALT with default setting (determined by cmake).