Added common include for simple C programs, and threads-intro code

include/common.h

@@ -0,0 +1,21 @@
+#ifndef __common_h__
+#define __common_h__
+
+#include <sys/time.h>
+#include <sys/stat.h>
+#include <assert.h>
+
+double GetTime() {
+    struct timeval t;
+    int rc = gettimeofday(&t, NULL);
+    assert(rc == 0);
+    return (double) t.tv_sec + (double) t.tv_usec/1e6;
+}
+
+void Spin(int howlong) {
+    double t = GetTime();
+    while ((GetTime() - t) < (double) howlong)
+	; // do nothing in loop
+}
+
+#endif // __common_h__