add callback functionality on certificate verification failure

Based on the differences to libmesode this functionality has been added.

It allows a library-user to set a callback for cases where the TLS stack
can't verify a received certificate and let the end-user decide what to
do.

examples/basic implements an example handler of said functionality.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

Makefile.am

@@ -48,6 +48,7 @@ libstrophe_la_SOURCES = \
 	src/snprintf.c \
 	src/sock.c \
 	src/stanza.c \
+	src/tls.c \
 	src/util.c \
 	src/uuid.c
 libstrophe_la_SOURCES += \