configure: use utilities from /usr/xpg4/bin if it exists

Solaris defaults to non-standard utilities (grep, sed, ...) with
proper ones being in /usr/xpg4/bin.  Prefixing PATH with this
directory when it exists ensures we get correct variants.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -54,6 +54,8 @@ if test "$E1" != 0 || test "$E2" = 0; then
     exit 1
 fi
 
+test -d /usr/xpg4/bin && PATH=/usr/xpg4/bin:$PATH
+
 show_help(){
     cat <<EOF
 Usage: configure [options]