Bugfix/c89 compile fixes (#327)

* Fix code to be able to compile with older C89 ANSI compilers

* Convert C++ comments to C89 comments.

* default to std=gnu89

* Fix to enable CMake 3.1 to build on Centos7

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/basic/object/nc.c

@@ -86,6 +86,7 @@ void Notification_Class_Property_Lists(
 void Notification_Class_Init(void)
 {
     uint8_t NotifyIdx = 0;
+    unsigned i;
 
     for (NotifyIdx = 0; NotifyIdx < MAX_NOTIFICATION_CLASSES; NotifyIdx++) {
         /* init with zeros */
@@ -99,7 +100,7 @@ void Notification_Class_Init(void)
         NC_Info[NotifyIdx].Priority[TRANSITION_TO_NORMAL] =
             255; /* PRINTF lowest priority for Normal message. */
         /* configure for every day, all day long */
-        for (unsigned i = 0; i < MAX_BACNET_DAYS_OF_WEEK; i++) {
+        for (i = 0; i < MAX_BACNET_DAYS_OF_WEEK; i++) {
             NC_Info[NotifyIdx].Recipient_List->ValidDays |= (1<<i);
         }
 		NC_Info[NotifyIdx].Recipient_List->FromTime.hour = 0;