build: fix libcrypto version linked by sort at runtime

build: fix libcrypto version linked by sort at runtime

One should link the versioned lib at runtime,
and the unversioned lib at build time,
as the unversioned lib may not be installed,
and better couples the binary with the required version.

* configure.ac: Define LIBCRYPTO_SONAME, determined from
the test binary linked with -lcrypto.  Also document
why we use SHA512() in the check, rather than MD5().
* src/sort.c (link_libcrypto): Use the versioned lib in dlopen().