add more choice for eigen downloading

cmake/external/eigen.cmake

@@ -7,8 +7,17 @@ INCLUDE_DIRECTORIES(${EIGEN_SOURCE_DIR}/src/eigen3)
 ExternalProject_Add(
     eigen3
     ${EXTERNAL_PROJECT_LOG_ARGS}
-    URL            "https://bitbucket.org/eigen/eigen/get/3.3.4.tar.gz"
-    URL_MD5        "1a47e78efe365a97de0c022d127607c3"
+    # for latest version, please get from official website
+    # URL            "https://bitbucket.org/eigen/eigen/get/3.3.4.tar.gz"
+    # URL_MD5        "1a47e78efe365a97de0c022d127607c3"
+
+    # for no-ssl http support, please get from bazel's mirror
+    # URL           "http://mirror.bazel.build/bitbucket.org/eigen/eigen/get/f3a22f35b044.tar.gz"
+    # URL_MD5       "4645c66075982da6fa0bcf6b20f3e8f7"
+
+    # get from github mirror
+    GIT_REPOSITORY  "https://github.com/RLovelett/eigen.git"
+    GIT_TAG         "a46d2e7337c4656f00abe54a8115f6d76153a048"
     PREFIX          ${EIGEN_SOURCE_DIR}
     UPDATE_COMMAND  ""
     CONFIGURE_COMMAND ""