The last time I built Cairo (from git master) was about 5 or 6 weeks
ago. Back then, quite a few of the source files contained a section like
this:-

       #if HAVE_CONFIG_H
       #include "config.h"
       #endif

but in today's version, the #if an #endif lines have gone - just
leaving  #include "config.h"

But where does config.h come from?  Is it generated by meson somehow?  I
don't use meson in my builds so I need to find out how to generate (or
obtain) config.h