Use cgo to get systems clock ticks for metrics

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

system/sysconfig.go

@@ -0,0 +1,13 @@
+// +build linux,cgo
+
+package system
+
+/*
+#include <unistd.h>
+int get_hz(void) { return sysconf(_SC_CLK_TCK); }
+*/
+import "C"
+
+func GetClockTicks() int {
+	return int(C.get_hz())
+}

system/sysconfig_nocgo.go

@@ -0,0 +1,9 @@
+// +build linux,!cgo
+
+package system
+
+func GetClockTicks() int {
+	// when we cannot call out to C to get the sysconf it is fairly safe to
+	// just return 100
+	return 100
+}

system/unsupported.go

@@ -17,3 +17,9 @@ func UsetCloseOnExec(fd uintptr) error {
 func Gettid() int {
 	return 0
 }
+
+func GetClockTicks() int {
+	// when we cannot call out to C to get the sysconf it is fairly safe to
+	// just return 100
+	return 100
+}