Since texinfo 6.8, there's no longer an INLINE_CONTENTS variable.

makeinfo: warning: set_from_init_file: unknown variable INLINE_CONTENTS

texinfo commit 62a6adfb33b006e187483779974bbd45f0f782b1 replaced
INLINE_CONTENTS with OUTPUT_CONTENTS_LOCATION.

texinfo commit 41f8ed4eb42bf6daa7df7007afd946875597452d replaced
OUTPUT_CONTENTS_LOCATION with CONTENTS_OUTPUT_LOCATION.

With texinfo 6.8 and above, the same as INLINE_CONTENTS=1 could be
achieved by CONTENTS_OUTPUT_LOCATION=inline.
https://www.gnu.org/software/texinfo/manual/texinfo/html_node/HTML-Customization-Variables.html