Fixed compilation error on Linux

CMakeLists.txt

@@ -290,9 +290,14 @@ INCLUDE_DIRECTORIES(BEFORE ${CMAKE_CURRENT_SOURCE_DIR}/src)
 
 SET(OPENMM_BUILD_SHARED_LIB ON CACHE BOOL "Whether to build shared OpenMM libraries")
 
+SET(EXTRA_LINK_FLAGS ${EXTRA_COMPILE_FLAGS})
+IF (CMAKE_SYSTEM_NAME MATCHES "Linux")
+    SET(EXTRA_LINK_FLAGS "${EXTRA_LINK_FLAGS} -lrt")
+ENDIF (CMAKE_SYSTEM_NAME MATCHES "Linux")
+
 IF(OPENMM_BUILD_SHARED_LIB)
     ADD_LIBRARY(${SHARED_TARGET} SHARED ${SOURCE_FILES} ${SOURCE_INCLUDE_FILES} ${API_ABS_INCLUDE_FILES})
-    SET_TARGET_PROPERTIES(${SHARED_TARGET} PROPERTIES LINK_FLAGS "${EXTRA_COMPILE_FLAGS}" COMPILE_FLAGS "${EXTRA_COMPILE_FLAGS} -DOPENMM_BUILDING_SHARED_LIBRARY -DLEPTON_BUILDING_SHARED_LIBRARY -DOPENMM_VALIDATE_BUILDING_SHARED_LIBRARY")
+    SET_TARGET_PROPERTIES(${SHARED_TARGET} PROPERTIES LINK_FLAGS "${EXTRA_LINK_FLAGS}" COMPILE_FLAGS "${EXTRA_COMPILE_FLAGS} -DOPENMM_BUILDING_SHARED_LIBRARY -DLEPTON_BUILDING_SHARED_LIBRARY -DOPENMM_VALIDATE_BUILDING_SHARED_LIBRARY")
     IF(WIN32)
         ADD_DEPENDENCIES(${SHARED_TARGET} PthreadsLibraries)
     ENDIF(WIN32)
@@ -302,7 +307,7 @@ SET(OPENMM_BUILD_STATIC_LIB OFF CACHE BOOL "Whether to build static OpenMM libra
 IF(OPENMM_BUILD_STATIC_LIB)
     ADD_LIBRARY(${STATIC_TARGET} STATIC ${SOURCE_FILES} ${SOURCE_INCLUDE_FILES} ${API_ABS_INCLUDE_FILES})
     SET(EXTRA_COMPILE_FLAGS "${EXTRA_COMPILE_FLAGS} -DOPENMM_USE_STATIC_LIBRARIES -DLEPTON_USE_STATIC_LIBRARIES -DPTW32_STATIC_LIB")
-    SET_TARGET_PROPERTIES(${STATIC_TARGET} PROPERTIES LINK_FLAGS "${EXTRA_COMPILE_FLAGS}" COMPILE_FLAGS "${EXTRA_COMPILE_FLAGS} -DOPENMM_BUILDING_STATIC_LIBRARY -DLEPTON_BUILDING_STATIC_LIBRARY -DOPENMMM_VALIDATE_BUILDING_STATIC_LIBRARY -DOPENMM_VALIDATE_BUILDING_STATIC_LIBRARY")
+    SET_TARGET_PROPERTIES(${STATIC_TARGET} PROPERTIES LINK_FLAGS "${EXTRA_LINK_FLAGS}" COMPILE_FLAGS "${EXTRA_COMPILE_FLAGS} -DOPENMM_BUILDING_STATIC_LIBRARY -DLEPTON_BUILDING_STATIC_LIBRARY -DOPENMMM_VALIDATE_BUILDING_STATIC_LIBRARY -DOPENMM_VALIDATE_BUILDING_STATIC_LIBRARY")
 ENDIF(OPENMM_BUILD_STATIC_LIB)
 
 IF(OPENMM_BUILD_C_AND_FORTRAN_WRAPPERS)