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/sys/ringbuf.h

@@ -69,6 +69,7 @@ extern "C" {
     /* tail */
     BACNET_STACK_EXPORT
     volatile void *Ringbuf_Peek(RING_BUFFER const *b);
+    BACNET_STACK_EXPORT
     bool Ringbuf_Pop(RING_BUFFER * b,
         uint8_t * data_element);
     BACNET_STACK_EXPORT
@@ -85,6 +86,7 @@ extern "C" {
     /* pair of functions to use head memory directly */
     BACNET_STACK_EXPORT
     volatile void *Ringbuf_Data_Peek(RING_BUFFER * b);
+    BACNET_STACK_EXPORT
     volatile void *Ringbuf_Peek_Next(RING_BUFFER const *b,
         uint8_t * data_element);
     BACNET_STACK_EXPORT