Fix POSIX conformance issues for clock_gettime

CLOCK_MONOTONIC should not be used as a feature macro.  The POSIX
feature macros are provided by unistd.h.  CLOCK_MONOTONIC is provided by
time.h.  CLOCK_MONOTONIC requires _POSIX_C_SOURCE >= 199309L on some
systems.

README.md

@@ -149,6 +149,8 @@ information on what to include when reporting a bug.
  - [X11] Bugfix: Decorations could not be enabled after window creation (#1566)
  - [X11] Bugfix: Content scale fallback value could be inconsistent (#1578)
  - [Wayland] Bugfix: The `GLFW_HAND_CURSOR` shape used the wrong image (#1432)
+ - [Wayland] Bugfix: `CLOCK_MONOTONIC` was not correctly enabled
+ - [POSIX] Bugfix: `CLOCK_MONOTONIC` was not correctly tested for or enabled
  - [NSGL] Removed enforcement of forward-compatible flag for core contexts
  - [NSGL] Bugfix: `GLFW_COCOA_RETINA_FRAMEBUFFER` had no effect on newer
    macOS versions (#1442)

src/posix_time.c

@@ -27,8 +27,11 @@
 // It is fine to use C99 in this file because it will not be built with VS
 //========================================================================
 
+#define _POSIX_C_SOURCE 199309L
+
 #include "internal.h"
 
+#include <unistd.h>
 #include <sys/time.h>
 #include <time.h>
 
@@ -41,7 +44,7 @@
 //
 void _glfwInitTimerPOSIX(void)
 {
-#if defined(CLOCK_MONOTONIC)
+#if defined(_POSIX_TIMERS) && defined(_POSIX_MONOTONIC_CLOCK)
     struct timespec ts;
 
     if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0)
@@ -64,7 +67,7 @@ void _glfwInitTimerPOSIX(void)
 
 uint64_t _glfwPlatformGetTimerValue(void)
 {
-#if defined(CLOCK_MONOTONIC)
+#if defined(_POSIX_TIMERS) && defined(_POSIX_MONOTONIC_CLOCK)
     if (_glfw.timer.posix.monotonic)
     {
         struct timespec ts;

src/wl_init.c

@@ -26,6 +26,8 @@
 // It is fine to use C99 in this file because it will not be built with VS
 //========================================================================
 
+#define _POSIX_C_SOURCE 199309L
+
 #include "internal.h"
 
 #include <assert.h>
@@ -38,6 +40,7 @@
 #include <sys/mman.h>
 #include <sys/timerfd.h>
 #include <unistd.h>
+#include <time.h>
 #include <wayland-client.h>
 
 

src/x11_init.c

@@ -36,6 +36,7 @@
 #include <limits.h>
 #include <stdio.h>
 #include <locale.h>
+#include <unistd.h>
 
 
 // Translate an X11 key code to a GLFW key code.