this should fix the MKL linking problem

CMakeLists.txt

@@ -179,6 +179,11 @@ if(MKL_FOUND AND GTSAM_WITH_EIGEN_MKL)
     set(EIGEN_USE_MKL_ALL 1) # This will go into config.h - it makes Eigen use MKL
     include_directories(${MKL_INCLUDE_DIR})
     list(APPEND GTSAM_ADDITIONAL_LIBRARIES ${MKL_LIBRARIES})
+    
+    # --no-as-needed is required with gcc according to the MKL link advisor
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+        set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--no-as-needed")  
+    endif()
 else()
     set(GTSAM_USE_EIGEN_MKL 0)
     set(EIGEN_USE_MKL_ALL 0)