That didn't work (I guess not surprisingly, as it leaves configmake.h included before system include files).
In any case, the reason for the two --include options is that in this project I'm mostly compiling Vala-generated C, and Vala is a bit inflexible with where it puts #include directives in its generated code. Specifically, there's no way to get it to put config.h at the top of the file. However, I don't need to have configmake.h on the command line like this (I can have it automatically added later by Vala); this was just my being lazy. So, I think I have a neat solution now.
Many thanks as always for your help.