Use posix timing to improve portability and avoid a build failure on
powerpc due to `__builtin_ppc_mftb' being gcc-specific.

Index: src/engine_c.c
--- src/engine_c.c.orig
+++ src/engine_c.c
@@ -884,6 +884,15 @@ void fixed_memory_ptrs_shut() {
 
   #if defined(__i386__) ||  defined(__x86_64__)
     #include <x86intrin.h>
+  #elif defined(_POSIX_MONOTONIC_CLOCK)
+    #include <time.h>
+    int64_t __rdtsc() {
+      struct timespec tp = {0};
+      int64_t timestamp = 0;
+      if (clock_gettime(CLOCK_MONOTONIC, &tp) == 0)
+        timestamp = tp.tv_sec * 1000000000 + tp.tv_nsec;
+      return timestamp;
+    }
   #elif defined(__powerpc__) || defined(__ppc__)
     #define __rdtsc() __builtin_ppc_mftb()
   #else