Second fix attempt for issue 349.

This is the only way to safely call clock_gettime (that I know of) when compiling for OSX 10.12 or iOS 10.
Refs #349.

src/civetweb.c

@@ -136,23 +136,11 @@ mg_static_assert(sizeof(void *) >= sizeof(int), "data type size check");
 #include <mach/mach_time.h>
 #include <assert.h>
 
-/* Determine if the current OSX version supports clock_gettime */
-#ifdef __APPLE__
-#include <AvailabilityMacros.h>
-#ifndef MAC_OS_X_VERSION_10_12
-#define MAC_OS_X_VERSION_10_12 101200
-#endif
-#endif
-#define CIVETWEB_APPLE_HAVE_CLOCK_GETTIME                                      \
-	defined(__APPLE__)                                                         \
-	    && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_12
-
-#if !(CIVETWEB_APPLE_HAVE_CLOCK_GETTIME)
 /* clock_gettime is not implemented on OSX prior to 10.12 */
-int clock_gettime(int clk_id, struct timespec *t);
+int _civet_clock_gettime(int clk_id, struct timespec *t);
 
 int
-clock_gettime(int clk_id, struct timespec *t)
+_civet_clock_gettime(int clk_id, struct timespec *t)
 {
 	memset(t, 0, sizeof(*t));
 	if (clk_id == CLOCK_REALTIME) {
@@ -192,7 +180,25 @@ clock_gettime(int clk_id, struct timespec *t)
 	}
 	return -1; /* EINVAL - Clock ID is unknown */
 }
+
+/* if clock_gettime is declared, then __CLOCK_AVAILABILITY will be defined */
+#ifdef __CLOCK_AVAILABILITY
+/* If we compiled with Mac OSX 10.12 or later, then clock_gettime will be declared
+ * but it may be NULL at runtime. So we need to check before using it. */
+int _civet_safe_clock_gettime(int clk_id, struct timespec *t);
+
+int
+_civet_safe_clock_gettime(int clk_id, struct timespec *t) {
+	if( clock_gettime ) {
+		return clock_gettime(clk_id, t);
+	}
+	return _civet_clock_gettime(clk_id, t);
+}
+#define clock_gettime _civet_safe_clock_gettime
+#else
+#define clock_gettime _civet_clock_gettime
 #endif
+
 #endif