Travis: add Windows build in a container

I expect this to become the default way to test Windows builds and
create installers on Travis. The idea is that instead of downloading the
pre-built MXE binaries we might as well use a container that has all
this installed and can be used locally to test if things fail on Travis;
which will allow us to have the exact same environment for testing
locally as runs on Travis.

At this point the container used is way too big - more effort needs to
be spent on shrinking it.

Right now this only deals with Subsurface and not with smtk2ssrf.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

scripts/windows-container/before_install.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+
+set -x
+set -e
+
+# when running this locally, set TRAVIS_BUILD_DIR to the Subsurface
+# directory inside your Windows build tree
+TRAVIS_BUILD_DIR=${TRAVIS_BUILD_DIR:-$PWD}
+
+git fetch --unshallow || true # if running locally, unshallow could fail
+git pull --tags
+git submodule init
+git describe
+
+# make sure we have libdivecomputer
+echo "Get libdivecomputer"
+cd ${TRAVIS_BUILD_DIR}
+git submodule update --recursive
+cd libdivecomputer
+autoreconf --install
+autoreconf --install
+
+# the intended layout as seen inside the container is
+# /win/subsurface # sources that we are testing
+#     /win32      # binaries that are build
+#     /grantlee
+#     /libzip
+#     /hidapi
+#     /googlemaps
+#
+# the first two are mounted as volumes (this way we get access to the
+# build results outside of the container
+cd ${TRAVIS_BUILD_DIR}/..
+mkdir -p win32
+
+# start the container and keep it running
+docker run -v $PWD/win32:/win/win32 -v $PWD/subsurface:/win/subsurface --name=builder -w /win -d dirkhh/mxe-build-container:0.6 /bin/sleep 60m
+
+# for some reason this package was installed but still isn't there?
+docker exec -t builder apt-get install -y ca-certificates
+
+# now set up our other dependencies
+# these are either not available in MXE, or a version that's too old
+docker exec -t builder bash subsurface/scripts/get-dep-lib.sh single . libzip
+docker exec -t builder bash subsurface/scripts/get-dep-lib.sh single . hidapi
+docker exec -t builder bash subsurface/scripts/get-dep-lib.sh single . googlemaps
+docker exec -t builder bash subsurface/scripts/get-dep-lib.sh single . grantlee
+
+
+# the rest we'll need when we enable smtk2ssrf
+
+#echo "Get mdbtools"
+#cd ${TRAVIS_BUILD_DIR}/..
+#git clone https://github.com/brianb/mdbtools.git
+
+# get prebuilt mxe libraries for mdbtools and glib.
+# do not overwrite upstream prebuilt mxe binaries if there is any coincidence.
+#wget https://www.dropbox.com/s/842skyusb96ii1u/mxe-static-minimal-994ad473.tar.xz
+#[[ ! -f mxe-static-minimal-994ad473.tar.xz ]] && exit 1
+#cd mxe
+#tar -xJf ../mxe-static-minimal-994ad473.tar.xz --skip-old-files
+#ls -al usr/