fix grep complain in configure: "warning: stray \ before c"

Signed-off-by: Alexander Golubev <fatzer2@gmail.com>
(cherry picked from commit 92fe774704)

configure

@@ -3462,14 +3462,14 @@ for file in .projects .projects.3; do
 	    continue;
 	fi
 	QMAKE_SPEC_ARGS="-spec $SPEC"
-	if echo '\c' | grep '\c' >/dev/null; then
+	if echo '\c' | grep '\\c' >/dev/null; then
 	    echo -n "  for $a"
 	else
 	    echo "  for $a\c"
 	fi
 
 	if [ "$file" = ".projects.3" ]; then
-	    if echo '\c' | grep '\c' >/dev/null; then
+	    if echo '\c' | grep '\\c' >/dev/null; then
 		echo -n " (fast)"
 	    else
 		echo " (fast)\c"
@@ -3493,7 +3493,7 @@ EOF
 	    EXEC="$outpath/bin/tqmake $QMAKE_SWITCHES $QMAKE_SPEC_ARGS -o $OUTDIR $a"
 	    if [ "$OPT_VERBOSE" = "yes" ]; then
 		echo " (`basename $SPEC`)"
-		if echo '\c' | grep '\c' >/dev/null; then
+		if echo '\c' | grep '\\c' >/dev/null; then
 		    echo -n "$EXEC"
 		else
 		    echo "$EXEC\c"