Improved handing of external rabbitmq-codegen

- By default look in the CMAKE_SOURCE_DIR/rabbitmq-codegen, then
CMAKE_SOURCE_DIR/../rabbitmq-codegen unless RABBITMQ_CODEGEN_DIR is set
- If FETCH_CODEGEN_FROM_GIT use the CMake ExternalProject to do it

CMakeLists.txt

@@ -19,23 +19,47 @@ if (CHECK_PYTHON_JSON_FAILED)
   endif (CHECK_PYTHON_SIMPLEJSON_FAILED)
 endif (CHECK_PYTHON_JSON_FAILED)
 
-
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_CURRENT_SOURCE_DIR})
 include(TestCInline)
 
-include(ExternalProject)
-ExternalProject_Add(
-	amqp_codegen
-	GIT_REPOSITORY https://github.com/rabbitmq/rabbitmq-codegen.git
-	GIT_TAG rabbitmq_v2_5_1
-	SOURCE_DIR amqp_codegen
-	CONFIGURE_COMMAND ""
-	BUILD_COMMAND ""
-	INSTALL_COMMAND ""
-	TEST_COMMAND "")
-
-ExternalProject_Get_Property(amqp_codegen SOURCE_DIR)
-set(AMQP_CODEGEN_DIR ${SOURCE_DIR})
+option(FETCH_CODEGEN_FROM_GIT "Fetch codegen directly from the git repository" OFF)
+
+set(CODEGEN_GIT_TAG rabbitmq_v2_5_1 CACHE STRING "Git tag in rabbitmq-codegen to fetch with FETCH_CODEGEN_FROM_GIT")
+if (FETCH_CODEGEN_FROM_GIT)
+    include(ExternalProject)
+    ExternalProject_Add(
+        amqp_codegen
+        GIT_REPOSITORY https://github.com/rabbitmq/rabbitmq-codegen.git
+        GIT_TAG ${CODEGEN_GIT_TAG}
+        SOURCE_DIR amqp_codegen
+        CONFIGURE_COMMAND ""
+        BUILD_COMMAND ""
+        INSTALL_COMMAND ""
+        TEST_COMMAND "")
+
+    ExternalProject_Get_Property(amqp_codegen SOURCE_DIR)
+    set(AMQP_CODEGEN_DIR ${SOURCE_DIR} CACHE PATH "Path to rabbitmq-codegen" FORCE)
+    message(STATUS "Using amqp_codegen.py in ${AMQP_CODEGEN_DIR}")
+else (FETCH_CODEGEN_FROM_GIT)
+    find_path(RABBITMQ_CODEGEN_DIR
+        amqp_codegen.py
+        PATHS ${CMAKE_SOURCE_DIR}/rabbitmq-codegen 
+              ${CMAKE_SOURCE_DIR}/../rabbitmq-codegen
+        DOC "Path to directory containing amqp_codegen.py (rabbitmq-codegen)"
+        NO_DEFAULT_PATH
+        )
+
+    if (RABBITMQ_CODEGEN_DIR STREQUAL "RABBITMQ_CODEGEN_DIR-NOTFOUND")
+        message(SEND_ERROR "Cannot find rabbitmq-codegen, set RABBITMQ_CODEGEN_DIR to a rabbitmq-codegen checkout, or set FETCH_CODEGEN_FROM_GIT to download it from git automatically")
+    else ()
+        message(STATUS "Using amqp_codegen.py in ${RABBITMQ_CODEGEN_DIR}")
+    endif()
+
+    set(AMQP_CODEGEN_DIR ${RABBITMQ_CODEGEN_DIR} CACHE PATH "Path to rabbitmq-codegen" FORCE)
+endif (FETCH_CODEGEN_FROM_GIT)
+
+mark_as_advanced(AMQP_CODEGEN_DIR)
+
 
 add_subdirectory(librabbitmq)
 add_subdirectory(examples)