Check that the the version of the library is at minimum the requested one
and return the version string; return NULL if the condition is not
satisfied. If a NULL is passed to this function, no check is done,
but the version string is simply returned.
See STRINGPREP_VERSION for a suitable req_version string.
RETURN VALUE
Version string of run-time library, or NULL if the
run-time library does not meet the required version number.
