Clearly document the OpenSSL version required

[blipper]

Is your feature request related to a problem? Please describe.
There are requirements for files/tests that are not present in earlier versions of OpenSSL.

Describe the solution you'd like
Document that exact minimum version needed.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[MMory]

Maybe we need to provide some internal dummy header? This only refers to the test cases of the type state analysis for openssl, for which we don't need any actual functionality of openssl.

[pdschubert]

Hi @meret-boe, please implement Martin's suggestion. For the test code in test/llvm_test_code/openssl/* various OpenSSL declarations are required to get it to compile to LLVM IR. We are currently including the respective header files from the user's system-wide OpenSSL installation. Let's create a dummy header instead that contains the necessary declarations such that we can remove the dependencies on OpenSSL.

Please create a dummy file for each original header file (cf. README.txt files).