Don't use CLOCK_UPTIME_RAW if it won't be defined (#16951)

This will fix #15957 by not trying to use CLOCK_UPTIME_RAW if the feature test macros are not currently set to make it available.

Since the build system is in charge of the feature test macros, and since this header might be included under any end user program's build system, we can't just #define the macros we would like.

Closes #16951

COPYBARA_INTEGRATE_REVIEW=#16951 from adamnovak:patch-1 cebf44e
PiperOrigin-RevId: 696251619

Author: adamnovak

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs

@@ -31,7 +31,8 @@
 #include "absl/memory/memory.h"
 #include "google/protobuf/message_lite.h"
 
-#if !defined(GOOGLE_PROTOBUF_NO_RDTSC) && defined(__APPLE__)
+#if !defined(GOOGLE_PROTOBUF_NO_RDTSC) && defined(__APPLE__) && \
+    (!defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE))
 #include <time.h>
 #endif
 
@@ -728,9 +729,10 @@ class PROTOBUF_EXPORT UntypedMapBase {
   map_index_t Seed() const {
     uint64_t s = 0;
 #if !defined(GOOGLE_PROTOBUF_NO_RDTSC)
-#if defined(__APPLE__)
+#if defined(__APPLE__) && \
+    (!defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE))
     // Use a commpage-based fast time function on Apple environments (MacOS,
-    // iOS, tvOS, watchOS, etc).
+    // iOS, tvOS, watchOS, etc), if we think the system headers expose it.
     s = clock_gettime_nsec_np(CLOCK_UPTIME_RAW);
 #elif defined(__x86_64__) && defined(__GNUC__)
     uint32_t hi, lo;