libraries/qt5: Add -legacy check to slackbuild.

No build bump required here.

libraries/qt5/qt5.SlackBuild

@@ -87,6 +87,13 @@ else
   LIBDIRSUFFIX=""
 fi
 
+if [ -e /var/log/packages/$PRGNAM-legacy-[[:digit:]]* ]; then
+  printf "%s\n%s\n%s\n" "You appear to have $PRGNAM-legacy installed." \
+    "$PRGNAM will not co-exist with $PRGNAM-legacy." \
+    "Please uninstall $PRGNAM-legacy and restart the SlackBuild."
+  exit 1
+fi
+
 set -e
 
 rm -rf $PKG