Bugfix/code clean using gcc warnings (#371)

* Enable extra GCC warnings to discover subtle bugs

* convert c++ comments to c comments

* cleanup pedantic compiler warnings

* Compile apps with GNU89 GNU99 GNU11 and GNU17

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

src/bacnet/basic/sys/platform.h

@@ -18,6 +18,12 @@
 #ifndef BACNET_SYS_PLATFORM_H
 #define BACNET_SYS_PLATFORM_H
 
+#include <math.h>
+
+#ifndef islessgreater
+#define islessgreater(x, y) ((x) < (y) || (x) > (y))
+#endif
+
 /* marking some code as 'deprecated' */
 # if defined(_MSC_VER)
 #   define BACNET_STACK_DEPRECATED(message) __declspec(deprecated(message))
@@ -31,4 +37,10 @@
 #  define strcasecmp _stricmp
 # endif
 
+#if defined(__GNUC__)
+#define BACNET_STACK_FALLTHROUGH() __attribute__ ((fallthrough))
+#else 
+#define BACNET_STACK_FALLTHROUGH() /* fall through */
+#endif
+
 #endif