fix compilation failure when makeinfo or texinfo are missing

doc/handbook/Makefile.am

doc/handbook/Makefile.in

doc/handbook/acheck-refman.texi

doc/handbook/acheck-specs.texi

doc/handbook/arc-commands.texi

doc/handbook/arc-handbook.texi

doc/handbook/arc-shell.texi

doc/handbook/display-ack-cmd.texi

doc/handbook/dot-ack-cmd.texi

doc/handbook/installation.texi

doc/handbook/macros.texi

doc/handbook/nrtest-ack-cmd.texi

doc/handbook/project-ack-cmd.texi

doc/handbook/quot-ack-cmd.texi

doc/handbook/references.texi

doc/handbook/show-ack-cmd.texi

doc/handbook/test-ack-cmd.texi

doc/handbook/wts-ack-cmd.texi