fix: timers on Linux

On Linux the clock() function measures cpu time instead of wall time. This change uses the Apple code path for Linux.

timer.c

@@ -1,6 +1,12 @@
 #include "timer.h"
 
-#if __APPLE__
+#ifdef __linux__
+// Defines strict posix compliance for CLOCK_MONOTONIC
+#define _POSIX_C_SOURCE 199309L
+#include <time.h>
+#endif
+
+#if __APPLE__ || __linux__
 #include <sys/time.h>
 static struct timespec start = {0, 0};
 
@@ -21,7 +27,8 @@ double stopTimer()
 
     double measuredSeconds = (double)delta_us / 1000000.;
 
-    if(start.tv_nsec > 0) {
+    if (start.tv_nsec > 0)
+    {
         start.tv_nsec = 0;
         start.tv_sec = 0;
     }