Add Debian sid build pass (#4707)

* Add Debian sid build pass

* Update file locations and copyright

* Use ccache

* Redirect within sudo

* Work around a pbuilder bug which breaks ccache

* Debian: enable parallel builds

* Travis: Debian: fix caching

* Fix shellcheck warnings

.travis/linux..script.sh

@@ -1,7 +1,14 @@
 #!/usr/bin/env bash
+mkdir build
+cd build
+
 if [ $QT5 ]; then
 	unset QTDIR QT_PLUGIN_PATH LD_LIBRARY_PATH
 	source /opt/qt59/bin/qt59-env.sh
 fi
 
 cmake -DUSE_WERROR=ON $CMAKE_FLAGS ..
+
+make -j4
+make tests
+./tests/tests