unix-ffi/machine/timer: Use libc if librt is not present.

Newer implementations of libc integrate the functions from librt,
for example glibc since 2.17 and uClibc-ng. So if the librt.so cannot
be loaded, it can be assumed that libc contains the expected functions.

Author: basvd

Diff for: unix-ffi/machine/machine/timer.py

@@ -5,7 +5,10 @@
 from signal import *
 
 libc = ffilib.libc()
-librt = ffilib.open("librt")
+try:
+    librt = ffilib.open("librt")
+except OSError as e:
+    librt = libc
 
 CLOCK_REALTIME = 0
 CLOCK_MONOTONIC = 1