Versioning engines default location: the Unix case

OpenSSL engines are tied to the OpenSSL shared library versions,
starting with OpenSSL 1.1.  We therefore need to install them in
directories which have the shared library version in it's name, to
easily allow multiple OpenSSL versions to be installed at the same
time.

For Unix, the default installation directory is changed from
$PREFIX/lib/engines to $PREFIX/lib/engines-${major}_${minor} (mingw)
or $PREFIX/lib/engines-${major}.${minor} (all but mingw)

($PREFIX is the directory given for the configuration option --prefix,
and ${major} and ${minor} are the major and minor shared library
version numbers)

Reviewed-by: Rich Salz <rsalz@openssl.org>