Add user can define PROTOBUF_ROOT.

.gitignore

@@ -16,3 +16,5 @@ third_party/
 *~
 bazel-*
 third_party/
+# for clion
+cmake-build-*

cmake/external/protobuf.cmake

@@ -14,6 +14,28 @@
 
 INCLUDE(ExternalProject)
 
+macro(PROMPT_PROTOBUF_LIB)
+    MESSAGE(STATUS "Protobuf protoc executable: ${PROTOBUF_PROTOC_EXECUTABLE}")
+    MESSAGE(STATUS "Protobuf library: ${PROTOBUF_LIBRARY}")
+    INCLUDE_DIRECTORIES(${PROTOBUF_INCLUDE_DIR})
+    RETURN()
+endmacro()
+
+set(PROTOBUF_ROOT "" CACHE PATH "Folder contains protobuf")
+if (NOT "${PROTOBUF_ROOT}" STREQUAL "")
+    find_path(PROTOBUF_INCLUDE_DIR google/protobuf/message.h PATHS ${PROTOBUF_ROOT}/include)
+    find_library(PROTOBUF_LIBRARY protobuf PATHS ${PROTOBUF_ROOT}/lib)
+    find_library(PROTOBUF_LITE_LIBRARY protobuf-lite PATHS ${PROTOBUF_ROOT}/lib)
+    find_library(PROTOBUF_PROTOC_LIBRARY protoc PATHS ${PROTOBUF_ROOT}/lib)
+    find_program(PROTOBUF_PROTOC_EXECUTABLE protoc PATHS ${PROTOBUF_ROOT}/bin)
+    if (PROTOBUF_INCLUDE_DIR AND PROTOBUF_LIBRARY AND PROTOBUF_LITE_LIBRARY AND PROTOBUF_PROTOC_LIBRARY)
+        message(STATUS "Using custom protobuf library in ${PROTOBUF_ROOT}.")
+        PROMPT_PROTOBUF_LIB()
+    else()
+        message(WARNING "Cannot find protobuf library in ${PROTOBUF_ROOT}.")
+    endif()
+endif()
+
 FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
     SET(PROTOBUF_SOURCES_DIR ${THIRD_PARTY_PATH}/${TARGET_NAME})
     SET(PROTOBUF_INSTALL_DIR ${THIRD_PARTY_PATH}/install/${TARGET_NAME})
@@ -107,6 +129,4 @@ IF(NOT PROTOBUF_FOUND)
     SET(PROTOBUF_PROTOC_LIBRARY ${protobuf_PROTOC_LIBRARY} CACHE FILEPATH "protoc library." FORCE)
 ENDIF(NOT PROTOBUF_FOUND)
 
-MESSAGE(STATUS "Protobuf protoc executable: ${PROTOBUF_PROTOC_EXECUTABLE}")
-MESSAGE(STATUS "Protobuf library: ${PROTOBUF_LIBRARY}")
-INCLUDE_DIRECTORIES(${PROTOBUF_INCLUDE_DIR})
+PROMPT_PROTOBUF_LIB()