doc/t2h.pm: fix missing TOC with texinfo 6.8 and above

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

doc/t2h.pm

@@ -131,7 +131,11 @@ my $program_version_num = version->declare(get_conf('PACKAGE_VERSION'))->numify;
 my $program_version_6_8 = $program_version_num >= 6.008000;
 
 # print the TOC where @contents is used
-set_from_init_file('INLINE_CONTENTS', 1);
+if ($program_version_6_8) {
+    set_from_init_file('CONTENTS_OUTPUT_LOCATION', 'inline');
+} else {
+    set_from_init_file('INLINE_CONTENTS', 1);
+}
 
 # make chapters <h2>
 set_from_init_file('CHAPTER_HEADER_LEVEL', 2);