POSIX: Remove use of deprecated gettimeofday

POSIX.1-2008 deprecated gettimeofday, which we used as a fallback if the
monotonic clock was unavailable.

This replaces that fallback with the non-monotonic real-time clock.

Because of the Gordian knot of feature test macros across Unices, this
also includes the shift from some platform source files defining
_POSIX_C_SOURCE to various values to _DEFAULT_SOURCE being defined for
all source files on Linux.  This is because -std=c99 on Linux disables
_DEFAULT_SOURCE (POSIX 2008 and extensions).

Once runtime platform selection comes in, this kind of platform-specific
preprocessor logic can be moved into the platform glue files and won't
need to be replicated by third-party build setups, but for now, sorry.

src/posix_time.h

@@ -28,13 +28,14 @@
 #define _GLFW_PLATFORM_LIBRARY_TIMER_STATE _GLFWtimerPOSIX posix
 
 #include <stdint.h>
+#include <time.h>
 
 
 // POSIX-specific global timer data
 //
 typedef struct _GLFWtimerPOSIX
 {
-    GLFWbool    monotonic;
+    clockid_t   clock;
     uint64_t    frequency;
 
 } _GLFWtimerPOSIX;