features_cpu, rcheevos: shim clock_gettime on pre-10.12 macOS

clock_gettime() was added to Darwin in macOS 10.12 Sierra / iOS 10.0
(2016).  On Leopard (10.5) and every other pre-10.12 macOS target,
the symbol doesn't exist in libSystem and the link fails:

  undefined reference to 'clock_gettime'

Two call sites in the current build:

  libretro-common/features/features_cpu.c:
    Already has an ra_clock_gettime shim that falls back to
    gettimeofday(), but the gate is iOS-only:
      #if __IPHONE_OS_VERSION_MIN_REQUIRED < 100000
    On macOS the shim is bypassed and ra_clock_gettime is
    #define'd to clock_gettime, producing the link error.

    FIX: introduce RA_NEEDS_CLOCK_GETTIME_SHIM defined when either
    iOS deployment target < 10.0 or macOS deployment target < 10.12,
    and use that flag instead of the iOS-only check.  Add the
    required #include <AvailabilityMacros.h>.

  deps/rcheevos/src/rc_client.c:
    Direct call in rc_client_clock_get_now_millisecs(), guarded by
    #if defined(CLOCK_MONOTONIC).  The function has a perfectly good
    #else fallback using clock()+time().

    FIX: #undef CLOCK_MONOTONIC at file scope on pre-10.12 macOS so
    the function's #if cascade falls through to the clock()+time()
    path.  <time.h> is transitively included through rc_api_info.h
    at the top of the file, so the #undef takes effect.  No rcheevos
    logic changed - this is a preprocessor-level selector flip.

Both gates use MIN_REQUIRED (deployment target) rather than
MAX_ALLOWED (SDK version).  That's correct here: Apple weak-links
clock_gettime when the deployment target is < 10.12, even when
compiling against a modern SDK where the declaration is visible.
The question is "can this binary run on a pre-10.12 system," which
is exactly what MIN_REQUIRED answers.

audio/librsound.c's clock_gettime call is already fenced behind a
__MACH__ check that routes macOS through host_get_clock_service
instead, so no change needed there.

No behavioral change on 10.12+ macOS, iOS 10.0+, or any other
platform: the shim is only compiled in when the deployment target
predates those versions.

Author: LibretroAdmin

deps/rcheevos/src/rc_client.c

@@ -21,6 +21,25 @@
   #endif
 #endif
 
+/* clock_gettime() was added to Darwin in iOS 10.0 / macOS 10.12.  On
+ * earlier deployment targets the symbol doesn't exist in libSystem
+ * and rc_client_clock_get_now_millisecs() below fails to link.
+ * Disable the CLOCK_MONOTONIC branch there so the function falls
+ * through to the clock()+time() #else fallback.  MIN_REQUIRED is
+ * the deployment target - what the binary must be able to run on -
+ * which is the right version to gate on here. */
+#if defined(__APPLE__) && defined(__MACH__)
+#include <AvailabilityMacros.h>
+#if (defined(MAC_OS_X_VERSION_MIN_REQUIRED) \
+       && MAC_OS_X_VERSION_MIN_REQUIRED < 101200) \
+ || (defined(__IPHONE_OS_VERSION_MIN_REQUIRED) \
+       && __IPHONE_OS_VERSION_MIN_REQUIRED < 100000)
+#ifdef CLOCK_MONOTONIC
+#undef CLOCK_MONOTONIC
+#endif
+#endif
+#endif
+
 #define RC_CLIENT_UNKNOWN_GAME_ID (uint32_t)-1
 #define RC_CLIENT_RECENT_UNLOCK_DELAY_SECONDS (10 * 60) /* ten minutes */
 

libretro-common/features/features_cpu.c

@@ -115,6 +115,7 @@
 /* iOS/OSX specific. Lacks clock_gettime(), so implement it. */
 #ifdef __MACH__
 #include <sys/time.h>
+#include <AvailabilityMacros.h>
 
 #ifndef CLOCK_MONOTONIC
 #define CLOCK_MONOTONIC 0
@@ -124,10 +125,18 @@
 #define CLOCK_REALTIME 0
 #endif
 
-/**
- * TODO/FIXME: clock_gettime function is part of iOS 10 now
- **/
-#if __IPHONE_OS_VERSION_MIN_REQUIRED < 100000
+/* clock_gettime() was added in iOS 10.0 / macOS 10.12 Sierra.
+ * On deployment targets below those versions the symbol doesn't
+ * exist in libSystem and the link fails, so fall back to a
+ * gettimeofday() shim.  Gated on MIN_REQUIRED (deployment target)
+ * rather than MAX_ALLOWED (SDK) because the binary needs to run on
+ * the deployment target, and Apple weak-links clock_gettime when
+ * targeting < 10.12 even from a newer SDK. */
+#if (defined(__IPHONE_OS_VERSION_MIN_REQUIRED) \
+       && __IPHONE_OS_VERSION_MIN_REQUIRED < 100000) \
+ || (defined(MAC_OS_X_VERSION_MIN_REQUIRED) \
+       && MAC_OS_X_VERSION_MIN_REQUIRED < 101200)
+#define RA_NEEDS_CLOCK_GETTIME_SHIM 1
 static int ra_clock_gettime(int clk_ik, struct timespec *t)
 {
    struct timeval now;
@@ -141,7 +150,7 @@ static int ra_clock_gettime(int clk_ik, struct timespec *t)
 #endif
 #endif
 
-#if defined(__MACH__) && __IPHONE_OS_VERSION_MIN_REQUIRED < 100000
+#if defined(__MACH__) && defined(RA_NEEDS_CLOCK_GETTIME_SHIM)
 #else
 #define ra_clock_gettime clock_gettime
 #endif