Avoid configure error when warning with no TERM

When configure wants to show a warning, it tries to check whether the
terminal supports colour.  This check produces an error message
when running with no TERM environment variable set (seen on a tinderbox)

Fix this check so that it tolerates failure of the `tput colors` command

Change-Id: I263136142120cf66fe900df0deb74d2fa6e5d1d0
Reviewed-on: https://gerrit.libreoffice.org/57009
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

configure.ac

@@ -112,7 +112,7 @@ add_warning()
     if test "$have_WARNINGS" = "no"; then
         echo "*************************************" > warn
         have_WARNINGS="yes"
-        if which tput >/dev/null 2>/dev/null && test `tput colors` -ge 8; then
+        if which tput >/dev/null && test "`tput colors 2>/dev/null || echo 0`" -ge 8; then
             dnl <esc> as actual byte (U+1b), [ escaped using quadrigraph @<:@
             COLORWARN='*@<:@1;33;40m WARNING @<:@0m:'
         else