texinfo: work around Solaris 10 xpg4 shell bug in install rules * lib/am/texinfos.am (install-html-am): Use an extra variable indirection to work around a bug in Solaris 10 /usr/xpg4/bin/sh. Bug revealed by a failure of `txinfo21.test'. See also: <http://lists.gnu.org/archive/html/bug-autoconf/2011-11/msg00005.html> <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10026#23>