Fix file path of default file in postinstall

postinstall.cmake.in

 endif()
 
 if(INIT_FLAVOR AND NOT EXISTS "$ENV{DESTDIR}${CONFIG_DIR}/@PROJECT_NAME@")
-	file(INSTALL FILES "@CMAKE_CURRENT_SOURCE_DIR@/@PROJECT_NAME@.default" DESTINATION "${CONFIG_DIR}" RENAME "@PROJECT_NAME@")
+	file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/@PROJECT_NAME@.default" DESTINATION "${CONFIG_DIR}" RENAME "@PROJECT_NAME@")
 endif()
 
 if(INIT_FLAVOR STREQUAL "sysv-lsb")