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

Since texinfo commit 6a5ceab6a48a4f052baad9f3474d741428409fd7, the
formatting functions, in particular begin_file, program_string and
end_file, are prefixed with format_, i.e. format_begin_file, etc.

This patch fixes building the documentation when texinfo 6.8, or
above, is used:

Unknown formatting type begin_file
 at /usr/bin/makeinfo line 415.
Unknown formatting type program_string
 at /usr/bin/makeinfo line 415.
Unknown formatting type end_file
 at /usr/bin/makeinfo line 415.

doc/t2h.pm

@@ -126,6 +126,10 @@ foreach my $command (keys(%Texinfo::Common::sectioning_commands), 'node') {
     texinfo_register_command_formatting($command, \&ffmpeg_heading_command);
 }
 
+# determine if texinfo is at least version 6.8
+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);
 
@@ -184,7 +188,11 @@ EOT
 
     return $head1 . $head_title . $head2 . $head_title . $head3;
 }
-texinfo_register_formatting_function('begin_file', \&ffmpeg_begin_file);
+if ($program_version_6_8) {
+    texinfo_register_formatting_function('format_begin_file', \&ffmpeg_begin_file);
+} else {
+    texinfo_register_formatting_function('begin_file', \&ffmpeg_begin_file);
+}
 
 sub ffmpeg_program_string($)
 {
@@ -201,7 +209,11 @@ sub ffmpeg_program_string($)
       $self->gdt('This document was generated automatically.'));
   }
 }
-texinfo_register_formatting_function('program_string', \&ffmpeg_program_string);
+if ($program_version_6_8) {
+    texinfo_register_formatting_function('format_program_string', \&ffmpeg_program_string);
+} else {
+    texinfo_register_formatting_function('program_string', \&ffmpeg_program_string);
+}
 
 # Customized file ending
 sub ffmpeg_end_file($)
@@ -220,7 +232,11 @@ EOT
 EOT
     return $program_text . $footer;
 }
-texinfo_register_formatting_function('end_file', \&ffmpeg_end_file);
+if ($program_version_6_8) {
+    texinfo_register_formatting_function('format_end_file', \&ffmpeg_end_file);
+} else {
+    texinfo_register_formatting_function('end_file', \&ffmpeg_end_file);
+}
 
 # Dummy title command
 # Ignore title. Title is handled through ffmpeg_begin_file().