[cmake] treat eigen and caffe2 includes as system

Eigen and Caffe2 are treating signedness loosely, which leads to
numerous warnings coming from their headers when compiling with "-Wall".
Mark includes from both Eigen and Caffe2 as SYSTEM in cmake to silence
the warnings triggered by those files as we are not in charge of fixing
them.

Author: ftynse

CMakeLists.txt

@@ -315,8 +315,8 @@ if (WITH_CAFFE2 AND WITH_CUDA)
    message(STATUS "Found, CAFFE2_INCLUDE_DIR: " ${CAFFE2_INCLUDE_DIR})
 
    add_definitions(-DCAFFE2_USE_GOOGLE_GLOG)
-   include_directories(AFTER ${EIGEN_INCLUDE_DIR})
-   include_directories(AFTER ${CAFFE2_INCLUDE_DIR})
+   include_directories(AFTER SYSTEM ${EIGEN_INCLUDE_DIR})
+   include_directories(AFTER SYSTEM ${CAFFE2_INCLUDE_DIR})
    add_subdirectory(tc/c2)
    add_subdirectory(tc/benchmarks)
    add_subdirectory(test/caffe2)