CMake: Add reminder for when we upgrade to C++17

dtool/src/dtoolbase/CMakeLists.txt

@@ -2,6 +2,13 @@ configure_file(pandaVersion.h.in pandaVersion.h)
 configure_file(checkPandaVersion.h.in checkPandaVersion.h)
 configure_file(checkPandaVersion.cxx.in checkPandaVersion.cxx)
 
+if(CMAKE_CXX_STANDARD GREATER_EQUAL 17)
+  # This serves as a reminder to update checkPandaVersion.h.in when we upgrade
+  # to C++17, which supports inline variables - a cleaner way of depending on
+  # the Panda version symbol from a header than what we're currently doing.
+  message(FATAL_ERROR "Developer notice: Update checkPandaVersion.h.in for C++17!")
+endif()
+
 set(P3DTOOLBASE_HEADERS
     ${CMAKE_CURRENT_BINARY_DIR}/checkPandaVersion.h
     ${CMAKE_CURRENT_BINARY_DIR}/pandaVersion.h