Build hook: Pass the timeout to the remote builder

Note that this will only work if the client has a very recent Nix
version (post 15e1b2c223), otherwise the
--option flag will just be ignored.

Fixes #50.

src/libstore/build.cc

@@ -690,10 +690,10 @@ HookInstance::HookInstance()
             if (dup2(builderOut.writeSide, 4) == -1)
                 throw SysError("dupping builder's stdout/stderr");
 
-            /* XXX: Pass `buildTimeout' to the hook?  */
             execl(buildHook.c_str(), buildHook.c_str(), settings.thisSystem.c_str(),
                 (format("%1%") % settings.maxSilentTime).str().c_str(),
                 (format("%1%") % settings.printBuildTrace).str().c_str(),
+                (format("%1%") % settings.buildTimeout).str().c_str(),
                 NULL);
 
             throw SysError(format("executing `%1%'") % buildHook);