Changed PROJECT_SOURCE_DIR to CMAKE_CURRENT_SOURCE_DIR to fix building when used as external project

CMakeLists.txt

@@ -13,8 +13,8 @@ if(MSVC)
 elseif(UNIX)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
-  execute_process(COMMAND ${SHELL_EXECUTABLE} -c sh autogen.sh WORKING_DIRECTORY ${PROJECT_SOURCE_DIR})
-  execute_process(COMMAND ${SHELL_EXECUTABLE} -c configure WORKING_DIRECTORY ${PROJECT_SOURCE_DIR})
+  execute_process(COMMAND ${SHELL_EXECUTABLE} -c sh autogen.sh WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+  execute_process(COMMAND ${SHELL_EXECUTABLE} -c configure WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
   file(COPY ./config.h DESTINATION ${CMAKE_CURRENT_BINARY_DIR}/include/)
   file(COPY ./json_config.h DESTINATION ${CMAKE_CURRENT_BINARY_DIR}/include/)
 endif()