need help with autoconf

I'm developing an extension to an existing plugin, which I will submit in due course back to the team.  However, I'm getting hung up on the autoconf-related aspects, so the Makefile.in files (for instance) are properly constructed to pass the right stuff to the compilation.  The issue is that the new code depends on a <xxxxx.h> header file, and I want to have the corresponding HAVE_XXXXX_H symbol defined.  I've tried looking at similar header files to see how they are treated, and experimenting a lot, but my

HAVE_XXXXX_H = @HAVE_XXXXX_H@

lines are not getting substituted as expected.  Is there something obvious I am missing?  Some special pixie dust I haven't spread around?  Some standard template for processing a header file I don't know about?  I've poked through the autoconf manual, added macro invocations that seem relevant, added a gl/m4/xxxxx_h.m4 macro file and a a reference to it, but nothing seems to do the trick.

Any guidance would be appreciated.

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