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/x11_init.c

@@ -1484,10 +1484,8 @@ void _glfwPlatformTerminate(void)
 const char* _glfwPlatformGetVersionString(void)
 {
     return _GLFW_VERSION_NUMBER " X11 GLX EGL OSMesa"
-#if defined(_POSIX_TIMERS) && defined(_POSIX_MONOTONIC_CLOCK)
-        " clock_gettime"
-#else
-        " gettimeofday"
+#if defined(_POSIX_MONOTONIC_CLOCK)
+        " monotonic"
 #endif
 #if defined(__linux__)
         " evdev"