Changes

dtls.c: Reduce logging for unsupported tls extensions

Every unsupported tls extension is logged using dtls_warn(), and the default
logging level is set to DTLS_LOG_WARN.

Update logging call to dtls_notice(), to reduce logging output noise.

Signed-off-by: Jon Shallow <supjps-libcoap@jpshallow.com>

dtls_prng_posix.c: Fix random() only support

Use of random() requires that _GNU_SOURCE is defined.
Cannot use variables named rand.

Make sure that dtls_prng.o is rebuilt whenever any of the
platform-specific/dtls_prng_*.c files are updated.

Signed-off-by: Jon Shallow <supjps-libcoap@jpshallow.com>