Fallback to os.clock() if computer.uptime() is not available

To be compatible with systems that don't have computer.uptime(), use
os.clock() as a fallback.  The time is only used for timeout
calculations.

bc.lua

@@ -1,5 +1,4 @@
 local uuid = require("uuid")
-local computer = require("computer")
 local bit32 = require("bit32")
 
 local Version = {0, 1}
@@ -31,6 +30,18 @@ local Query = {
   Below         = setmetatable({ty=0x424c4f, invalid=true}, query_param),
 }
 
+-- Helpers ---------------------------------------------------------------- {{{
+local clock -- A monotonically increasing timer for timeout calculation
+do
+  local status, computer = pcall(require, "computer")
+  if status and computer.uptime ~= nil then
+    clock = computer.uptime
+  else
+    clock = os.clock
+  end
+end
+-- }}}
+
 -- Network ---------------------------------------------------------------- {{{
 local Network = {
   default_port = bit32.bor(0xBC00, bit32.lshift(Version[1], 4), Version[2]),
@@ -190,11 +201,11 @@ function BaseControl:finalize(waits, timeout)
       end
     end
 
-    local tstart = computer.uptime()
+    local tstart = clock()
     local timeout_remaining = timeout
     while num_remaining > 0 do
       if timeout ~= nil then
-        timeout_remaining = timeout - (computer.uptime() - tstart)
+        timeout_remaining = timeout - (clock() - tstart)
         if timeout_remaining <= 0 then
           error("timeout")
         end