Add an option to control building static library with PIC or not

The option WITH_PIC is default to OFF.

By default, the static library is built from the
source code directly. If WITH_PIC is enabled, the
static library is built with the same set of
object libraries that the shared library uses.

To link Mosquitto static library into a shared
library, one must enable WITH_PIC to make the
piece of code locatable.

Signed-off-by: Lance Chen <cyen0312@gmail.com>