Update GetGitRevisionDescription.cmake

From https://github.com/rpavlik/cmake-modules

cmake/modules/GetGitRevisionDescription.cmake

@@ -40,7 +40,7 @@ set(__get_git_revision_description YES)
 get_filename_component(_gitdescmoddir ${CMAKE_CURRENT_LIST_FILE} PATH)
 
 function(get_git_head_revision _refspecvar _hashvar)
-	set(GIT_PARENT_DIR "${CMAKE_SOURCE_DIR}")
+	set(GIT_PARENT_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
 	set(GIT_DIR "${GIT_PARENT_DIR}/.git")
 	while(NOT EXISTS "${GIT_DIR}")	# .git dir not found, search parent directories
 		set(GIT_PREVIOUS_PARENT "${GIT_PARENT_DIR}")
@@ -53,6 +53,13 @@ function(get_git_head_revision _refspecvar _hashvar)
 		endif()
 		set(GIT_DIR "${GIT_PARENT_DIR}/.git")
 	endwhile()
+	# check if this is a submodule
+	if(NOT IS_DIRECTORY ${GIT_DIR})
+		file(READ ${GIT_DIR} submodule)
+		string(REGEX REPLACE "gitdir: (.*)\n$" "\\1" GIT_DIR_RELATIVE ${submodule})
+		get_filename_component(SUBMODULE_DIR ${GIT_DIR} PATH)
+		get_filename_component(GIT_DIR ${SUBMODULE_DIR}/${GIT_DIR_RELATIVE} ABSOLUTE)
+	endif()
 	set(GIT_DATA "${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/git-data")
 	if(NOT EXISTS "${GIT_DATA}")
 		file(MAKE_DIRECTORY "${GIT_DATA}")