Export symbols in order to support shared libraries (#54)

* Add BAC_ROUTING compile definition

* Reorder cmake

* Fix OpenSSL support: support both 1.0 and 1.1t pus

* Explicitly export symbols, hidden by default

* Build shared libraries on travis using cmake

* Learn Makefile about static library

* Fix build using mingw with cmake

* Do not cleanup twice or after potential free

ports/linux/bacport.h

@@ -99,10 +99,12 @@
 #include <sys/un.h>
 #include <sys/ioctl.h>
 #include <netdb.h>
+#include "bacnet/bacnet_stack_exports.h"
 
 /** @file linux/net.h  Includes Linux network headers. */
 
 /* Local helper functions for this port */
+BACNET_STACK_EXPORT
 extern int bip_get_local_netmask(
     struct in_addr *netmask);