Added check for makeinfo version >= 5.0.

configure.ac

@@ -90,6 +90,25 @@ else
 fi
 AM_CONDITIONAL([HAVE_HELP2MAN], [test x$HELP2MAN = xhelp2man])
 
+# Check for makeinfo version >= 5, required for building documentation.
+AC_PROG_SED
+AC_CHECK_PROG([MAKEINFO_FOUND], [makeinfo], [yes])
+if test x"${MAKEINFO_FOUND}" = xyes
+then
+  MAKEINFO_VERSION_REQ=5
+  AC_MSG_CHECKING([makeinfo version >= $MAKEINFO_VERSION_REQ])
+  MAKEINFO_VERSION=`makeinfo --version | sed -ne 's/^makeinfo .* \([[0-9]]\+\)\.[[0-9]]\+/\1/p'`
+  if test x$MAKEINFO_VERSION = x -o 0$MAKEINFO_VERSION -lt $MAKEINFO_VERSION_REQ
+  then
+    AC_MSG_RESULT([no])
+    AC_MSG_ERROR([Program 'makeinfo' version >= $MAKEINFO_VERSION_REQ is required.])
+  else
+    AC_MSG_RESULT([yes])
+  fi
+else
+  AC_MSG_ERROR([Missing program 'makeinfo', please install.)])
+fi
+
 # Check if we have Python installed
 AM_PATH_PYTHON([2.6],, [:])
 AM_CONDITIONAL([HAVE_PYTHON], [test "$PYTHON" != :])