>
> .../c++  ...  /opt/local/lib/libssl.dylib
>

I get the same, libssl from /opt/local, so we are not using openssl shipped with mac os.

We still do not know were OP's libssl comes from and if there is a mismatch between
openssl header files and library (header files from one openssl, library from a different openssl).

OP's cmake output: (no VERBOSE=1)
> -- MIDAS: cmake version: 3.16.3
> -- MIDAS: Found OpenSSL version 1.1.1d

Stefan's and my cmake output: (VERBOSE=1)
> -- MIDAS: cmake version: 3.16.3
> -- Found OpenSSL: /opt/local/lib/libcrypto.dylib (found version "1.1.1d")  
> -- MIDAS: Found OpenSSL version 1.1.1d

Without VERBOSE=1 cmake does not tell is which OpenSSL it found (not useful).
With VERBOSE=1 cmake outputs a flood of gunk (not useful).

My solution is to filter the cmake output with grep in "make cmake" ("make cmake3").

So please use that: "make cmake" - the output is roughly the same as normal make - compiler command lines (CFLAGS, library paths, etc),
compiler error message and all of the useful cmake output.

K.O.