build-system: Docker build for 64bit MXE

We previously tried to build the MXE Docker container on GitHub using
an Action, but that really didn't work well and was a lot more trouble
than it was worth.

So this goes back to an offline build mechanism where I simply create
an updated Docker image when needed and push that to Docker Hub.

But this nearly hides the most interesting change here - we are finally
switching to using 64bit binaries on Windows. It's 2020 and fewer than
1% of our users use 32bit Windows machines. We'll need to expand this
to be able to have both a 32bit and a 64bit version of Subsurface for
Windows. But for now, this solves the problem for 99% of our users.

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

scripts/docker/mxe-build-container/settings.mk

@@ -0,0 +1,44 @@
+# This is a template of configuration file for MXE. See
+# index.html for more extensive documentations.
+
+# This variable controls the number of compilation processes
+# within one package ("intra-package parallelism").
+JOBS := 8
+
+# This variable controls the targets that will build.
+MXE_TARGETS :=  x86_64-w64-mingw32.shared
+
+# The three lines below makes `make` build these "local packages" instead of all packages.
+# The ordering of the list appears weird, but this seems to help to get the build done
+# faster on a massively parallel machine to get some of the bottleneck packages built as
+# early as possible
+LOCAL_PKG_LIST := gcc \
+                  openssl \
+                  libmysqlclient \
+                  postgresql \
+                  qtbase \
+                  qtwebkit \
+                  nsis \
+                  curl \
+                  libxml2 \
+                  libxslt \
+                  libzip \
+                  libusb1 \
+                  hidapi \
+                  libgit2 \
+                  libftdi1 \
+                  mdbtools \
+                  qtconnectivity \
+                  qtdeclarative \
+                  qtimageformats \
+                  qtlocation \
+                  qtmultimedia \
+                  qtquickcontrols \
+                  qtquickcontrols2 \
+                  qtcharts \
+                  qtsvg \
+                  qttools \
+                  qttranslations \
+                  zstd
+.DEFAULT local-pkg-list:
+local-pkg-list: $(LOCAL_PKG_LIST)