It seems we require texinfo 4.7 for the --plaintext option, so
document that and check for the correct version in configure.
ChangeLog:
2014-12-15 Will Newton <will.newton@linaro.org>
* manual/install.texi: Bump required version of texinfo
to 4.7 from 4.5.
* INSTALL: Regenerated.
* configure.ac: Check for makeinfo version 4.7 and above.
* configure: Regenerated.