Set protobuf version when PROTOBUF_ROOT set

cmake/external/protobuf.cmake

@@ -17,9 +17,14 @@ INCLUDE(ExternalProject)
 macro(PROMPT_PROTOBUF_LIB)
     MESSAGE(STATUS "Protobuf protoc executable: ${PROTOBUF_PROTOC_EXECUTABLE}")
     MESSAGE(STATUS "Protobuf library: ${PROTOBUF_LIBRARY}")
+    MESSAGE(STATUS "Protobuf version: ${PROTOBUF_VERSION}")
     INCLUDE_DIRECTORIES(${PROTOBUF_INCLUDE_DIR})
     RETURN()
 endmacro()
+macro(SET_PROTOBUF_VERSION)
+    EXEC_PROGRAM(${PROTOBUF_PROTOC_EXECUTABLE} ARGS --version OUTPUT_VARIABLE PROTOBUF_VERSION)
+    STRING(REGEX MATCH "[0-9]+.[0-9]+" PROTOBUF_VERSION "${PROTOBUF_VERSION}")
+endmacro()
 
 set(PROTOBUF_ROOT "" CACHE PATH "Folder contains protobuf")
 if (NOT "${PROTOBUF_ROOT}" STREQUAL "")
@@ -30,6 +35,7 @@ if (NOT "${PROTOBUF_ROOT}" STREQUAL "")
     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 AND PROTOBUF_PROTOC_EXECUTABLE)
         message(STATUS "Using custom protobuf library in ${PROTOBUF_ROOT}.")
+        SET_PROTOBUF_VERSION()
         PROMPT_PROTOBUF_LIB()
     else()
         message(WARNING "Cannot find protobuf library in ${PROTOBUF_ROOT}.")
@@ -100,8 +106,7 @@ IF(NOT CMAKE_CROSSCOMPILING)
     FIND_PACKAGE(Protobuf ${PROTOBUF_VERSION})
 
     IF(PROTOBUF_FOUND)
-        EXEC_PROGRAM(${PROTOBUF_PROTOC_EXECUTABLE} ARGS --version OUTPUT_VARIABLE PROTOBUF_VERSION)
-        STRING(REGEX MATCH "[0-9]+.[0-9]+" PROTOBUF_VERSION "${PROTOBUF_VERSION}")
+        SET_PROTOBUF_VERSION()
         IF("${PROTOBUF_VERSION}" VERSION_LESS "3.1.0")
             SET(PROTOBUF_FOUND OFF)
         ENDIF()