See GSASL_VERSION for a suitable req_version string.
This function is one of few in the library that can be used without
a successful call to gsasl_init().
Check that the version of the library is at
minimum the one given as a string in req_version and return the
actual version string of the library; return NULL if the
condition is not met. If NULL is passed to this function no
check is done and only the version string is returned.