XCode v8 introduced clock_gettime() for iOS v10. However, it is a weak symbol, which means when earlier iOS versions try to use clock_gettime() it results in a crash due to the missing symbol. Detect this condition and do not set HAVE_CLOCK_GETTIME_MONOTONIC.

CMakeLists.txt

@@ -64,6 +64,18 @@ return 0;
 	}
 	"
 	IOS)
+
+	CHECK_C_SOURCE_COMPILES ("
+	#include <stdio.h>
+	#include <TargetConditionals.h>
+	int main() {
+#if TARGET_OS_IPHONE == 0 || __IPHONE_OS_VERSION_MIN_REQUIRED < 100000
+#error Not iOS v10
+#endif
+return 0;
+	}
+	"
+	IOS_V10)
 ENDIF ()
 
 IF (IOS AND HAVE_LIBRESOLV)
@@ -236,7 +248,13 @@ CHECK_SYMBOL_EXISTS (SIOCGIFADDR     "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_IOCTL_S
 CHECK_SYMBOL_EXISTS (MSG_NOSIGNAL    "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_MSG_NOSIGNAL)
 CHECK_SYMBOL_EXISTS (PF_INET6        "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_PF_INET6)
 CHECK_SYMBOL_EXISTS (SO_NONBLOCK     "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_SO_NONBLOCK)
-CHECK_SYMBOL_EXISTS (CLOCK_MONOTONIC "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_CLOCK_GETTIME_MONOTONIC)
+
+# XCode v8 bug: iOS when targeting less than v10 will say clock_gettime exists, it is a weak
+# symbol that only exists in iOS10 and will cause a crash at runtime when running on versions
+# less than 10.  Skip finding CLOCK_MONOTONIC on iOS target < 10.
+IF ((NOT IOS) OR IOS_V10)
+	CHECK_SYMBOL_EXISTS (CLOCK_MONOTONIC "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_CLOCK_GETTIME_MONOTONIC)
+ENDIF ()
 
 CHECK_STRUCT_HAS_MEMBER("struct sockaddr_in6" sin6_scope_id "${CMAKE_EXTRA_INCLUDE_FILES}" HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID LANGUAGE C)