Cant find librt while running sim_vehicle cmmand

hi i have installed everything on ubuntu 16.04 xenial
when i try to run the sim_vehicle form ArduPLane following eror comes:
SIM_VEHICLE: Killing tasks
SIM_VEHICLE: Starting up at -35.363261,149.165230,584,353 (CMAC)
SIM_VEHICLE: Configure waf
SIM_VEHICLE: “/opt/ardupilot/modules/waf/waf-light” “configure” “–board” “sitl”
Setting top to : /opt/ardupilot
Setting out to : /opt/ardupilot/build
Autoconfiguration : enabled
GCOV code coverage analysis : no
Setting board to : sitl
Using toolchain : native
Checking for ‘g++’ (C++ compiler) : /usr/bin/g++
Checking for ‘gcc’ (C compiler) : /usr/bin/gcc
Checking for c flags ‘-MMD’ : no
Checking for cxx flags ‘-MMD’ : no
Checking for need to link with librt : necessary
Checking for library rt : not found
The configuration failed
(complete log in /opt/ardupilot/build/config.log)
SIM_VEHICLE: (Configure waf) exited with code 256
SIM_VEHICLE: Killing tasks

also my jsbsim is not making successfully following error happens:
Making all in src
make[1]: Entering directory ‘/opt/jsbsim/src’
Making all in initialization
make[2]: Entering directory ‘/opt/jsbsim/src/initialization’
CXX libInit_la-FGSimplexTrim.lo
In file included from …/…/src/math/FGStateSpace.h:27:0,
from FGSimplexTrim.h:23,
from FGSimplexTrim.cpp:20:
…/…/src/models/propulsion/FGTurbine.h:297:3: error: ‘FGParameter’ does not name a type
FGParameter *N1SpoolUp;
…/…/src/models/propulsion/FGTurbine.h:298:3: error: ‘FGParameter’ does not name a type
FGParameter *N1SpoolDown;
…/…/src/models/propulsion/FGTurbine.h:299:3: error: ‘FGParameter’ does not name a type
FGParameter *N2SpoolUp;
…/…/src/models/propulsion/FGTurbine.h:300:3: error: ‘FGParameter’ does not name a type
FGParameter *N2SpoolDown;
…/…/src/models/propulsion/FGTurbine.h:310:1: error: expected class-name before ‘{’ token
Makefile:487: recipe for target ‘libInit_la-FGSimplexTrim.lo’ failed
make[2]: *** [libInit_la-FGSimplexTrim.lo] Error 1
make[2]: Leaving directory ‘/opt/jsbsim/src/initialization’
Makefile:612: recipe for target ‘all-recursive’ failed
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory ‘/opt/jsbsim/src’
Makefile:387: recipe for target ‘all-recursive’ failed
make: *** [all-recursive] Error 1

kindly solve these issues as my project deadline is very near
I installed ardupilot
then jsbsim
then confirgured ardupilot for sitl by running following commands:
Ardupilot build was successful for plane and copter which means folowing commands run successfully:

./waf configure --board sitl

./waf copter

./waf plane

but even then i am getting top errors kindly help asap