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/binding/address.c

@@ -302,7 +302,7 @@ bool address_mac_from_ascii(BACNET_MAC_ADDRESS *mac, const char *arg)
 {
     unsigned a[6] = { 0 }, p = 0;
     uint16_t port = 0;
-    int c;
+    int c, i;
     bool status = false;
 
     if (!(mac && arg)) {
@@ -327,7 +327,7 @@ bool address_mac_from_ascii(BACNET_MAC_ADDRESS *mac, const char *arg)
             &a[4], &a[5]);
 
         if (c > 0) {
-            for (int i = 0; i < c; i++) {
+            for (i = 0; i < c; i++) {
                 mac->adr[i] = a[i];
             }
             mac->len = c;