android: add script in Android archive to create a SDK

Also,
- Move mingw's pkg-support into build-scripts
- Add type annotations to python scripts for mypy
- ci: use v4 tag of actions/cache
- cmake: use PYTHON3_EXECUTABLE for running xxd.py
  (Python3::Interpreter is not always recognized.)

build-scripts/pkg-support/mingw/Makefile

@@ -0,0 +1,37 @@
+#
+# Makefile for installing the mingw32 version of the SDL library
+
+DESTDIR = /usr/local
+ARCHITECTURES := i686-w64-mingw32 x86_64-w64-mingw32
+
+default:
+	@echo "Run \"make install-i686\" to install 32-bit"
+	@echo "Run \"make install-x86_64\" to install 64-bit"
+	@echo "Run \"make install-all\" to install both"
+	@echo "Add DESTDIR=/custom/path to change the destination folder"
+
+install:
+	@if test -d $(ARCH) && test -d $(DESTDIR); then \
+		(cd $(ARCH) && cp -rv bin include lib share $(DESTDIR)/); \
+	else \
+		echo "*** ERROR: $(ARCH) or $(DESTDIR) does not exist!"; \
+		exit 1; \
+	fi
+
+install-i686:
+	$(MAKE) install ARCH=i686-w64-mingw32
+
+install-x86_64:
+	$(MAKE) install ARCH=x86_64-w64-mingw32
+
+install-all:
+	@if test -d $(DESTDIR); then \
+		for arch in $(ARCHITECTURES); do \
+			$(MAKE) install ARCH=$$arch DESTDIR=$(DESTDIR)/$$arch; \
+		done \
+	else \
+		echo "*** ERROR: $(DESTDIR) does not exist!"; \
+		exit 1; \
+	fi
+
+.PHONY: default install install-i686 install-x86_64 install-all