CMAKE_POSITION_INDEPENDENT_CODE should be optional about liboqs

[SWilson4]

I dug through the git blame to see why we would have hardcoded this, and it led me to PR #670, citing OpenSSL build failures. I believe that PR predated oqs-provider, so presumably it refers to our deprecated OpenSSL fork. Any idea if hardcoding the option is still necessary @baentsch @christianpaquin?

[baentsch]

Good "dig" @SWilson4 . Yes, the rationale given by @kevinmkane in 2020 seems sensible when building liboqs embedded into openssl -- and the PR definitely predates oqsprovider... which typically is a shared lib, though... ;-) But seriously, AFAIK oqsprovider can be configured to be built statically as well as shared, so hard-coding -fPIC does not seem a pre-requisite any more these days (and also shouldn't be).