sslgen.c: add compile-time check for SSL-backend completeness

#ifdef USE_SSL

/* SSL 'backend' compile-time sanity checks */

#if !defined(curlssl_init)

# error "SSL backend lacks definition for curlssl_init"

#elif !defined(curlssl_cleanup)

# error "SSL backend lacks definition for curlssl_cleanup"

#elif !defined(curlssl_connect) && !defined(curlssl_connect_nonblocking)

# error "SSL backend lacks curlssl_connect or curlssl_connect_nonblocking"

#elif !defined(curlssl_session_free)

# error "SSL backend lacks definition for curlssl_session_free"

#elif !defined(curlssl_close_all)

# error "SSL backend lacks definition for curlssl_close_all"

#elif !defined(curlssl_close)

# error "SSL backend lacks definition for curlssl_close"

#elif !defined(curlssl_shutdown)

# error "SSL backend lacks definition for curlssl_shutdown"

#elif !defined(curlssl_set_engine)

# error "SSL backend lacks definition for curlssl_set_engine"

#elif !defined(curlssl_set_engine_default)

# error "SSL backend lacks definition for curlssl_set_engine_default"

#elif !defined(curlssl_engines_list)

# error "SSL backend lacks definition for curlssl_engines_list"

#elif !defined(curlssl_version)

# error "SSL backend lacks definition for curlssl_version"

#elif !defined(curlssl_check_cxn)

# error "SSL backend lacks definition for curlssl_check_cxn"

#elif !defined(curlssl_data_pending)

# error "SSL backend lacks definition for curlssl_data_pending"

#elif !defined(curlssl_random)

# error "SSL backend lacks definition for curlssl_random"

#elif !defined(curlssl_md5sum)

# error "SSL backend lacks definition for curlssl_md5sum"

#endif

/* "global" init done? */

static bool init_ssl=FALSE;