[WIP] Switch MinGW building to CMake

Automatically grabs and compiles libpng, and avoids having hacky lines in our Makefile
(the compiler specification *should* be orthogonal to the build target!)

Also move the MinGW package install to the `install_deps.sh` script,
to move logic off of the YAML.

Author: ISSOtm

.github/scripts/install_deps.sh

@@ -1,11 +1,35 @@
 #!/bin/sh
 # This script requires `sh` instead of `bash` because the latter is not always installed on FreeBSD.
-set -eu
+set -eux
 
-case "${1%-*}" in
+USAGE="Usage: $0 <os> [additional toolset]"
+
+OS="${1:?$USAGE}"
+shift
+TOOLSET="${1-}"
+shift 2>/dev/null || : # That argument is optional.
+if [ $# -ne 0 ]; then
+	echo >&2 "$USAGE"
+	exit 1
+fi
+
+case "${OS%-*}" in
 	ubuntu)
+		pkgs=bison
+		case "$TOOLSET" in
+			mingw32) TOOLSET=
+				pkgs="$pkgs g++-mingw-w64-i686-win32   mingw-w64-tools libz-mingw-w64-dev dpkg-dev"
+				;;
+			mingw64) TOOLSET=
+				pkgs="$pkgs g++-mingw-w64-x86-64-win32 mingw-w64-tools libz-mingw-w64-dev dpkg-dev"
+				;;
+			'')
+				pkgs="$pkgs libpng-dev"
+				;;
+		esac
 		sudo apt-get -qq update
-		sudo apt-get install -yq bison libpng-dev pkg-config
+		#shellcheck disable=SC2086 # (This word splitting is intentional.)
+		sudo apt-get install -yq $pkgs
 		;;
 	macos)
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.
@@ -30,6 +54,11 @@ case "${1%-*}" in
 		;;
 esac
 
+if [ -n "$TOOLSET" ]; then
+	printf >&2 'Unknown toolset `%s` for OS `%s`\n' "$TOOLSET" "$OS"
+	exit 1
+fi
+
 echo "PATH=($PATH)" | sed 's/:/\n      /g'
 bison --version
 make --version

.github/workflows/testing.yml

@@ -240,12 +240,12 @@ jobs:
       - name: Install MinGW
         run: | # dpkg-dev is apparently required for pkg-config for cross-building
           sudo apt-get install g++-mingw-w64-${{ matrix.arch }}-win32 mingw-w64-tools libz-mingw-w64-dev dpkg-dev
-      - name: Install libpng dev headers for MinGW
-        run: |
-          ./.github/scripts/mingw-w64-libpng-dev.sh ${{ matrix.triplet }}
+      #- name: Install libpng dev headers for MinGW
+      #  run: |
+      #    ./.github/scripts/mingw-w64-libpng-dev.sh ${{ matrix.triplet }}
       - name: Cross-build Windows binaries
         run: |
-          make mingw${{ matrix.bits }} -kj Q=
+          cmake -B build -DCMAKE_TOOLCHAIN_FILE=cmake/toolchain-mingw${{ matrix.bits }}.cmake
       - name: Package binaries
         run: | # DLL dependencies can be figured out using e.g. Dependency Walker or objdump -p
           mkdir bins

.gitignore

@@ -14,6 +14,6 @@ CMakeCache.txt
 CMakeFiles/
 cmake_install.cmake
 CMakeUserPresets.json
-build/
+build*/
 *.dSYM/
 callgrind.out.*

cmake/toolchain-mingw32.cmake

@@ -0,0 +1,18 @@
+# From https://www.mingw-w64.org/build-systems/cmake/
+
+set(CMAKE_SYSTEM_NAME Windows)
+set(CMAKE_SYSTEM_PROCESSOR i686)
+
+# specify the cross compiler
+set(CMAKE_C_COMPILER i686-w64-mingw32-gcc)
+set(CMAKE_CXX_COMPILER i686-w64-mingw32-g++)
+set(CMAKE_RC_COMPILER i686-w64-mingw32-windres)
+
+# where is the target environment
+set(CMAKE_FIND_ROOT_PATH /usr/i686-w64-mingw32)
+
+# search for programs in the build host directories
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+# for libraries and headers in the target directories
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

cmake/toolchain-mingw64.cmake

@@ -0,0 +1,18 @@
+# From https://www.mingw-w64.org/build-systems/cmake/
+
+set(CMAKE_SYSTEM_NAME Windows)
+set(CMAKE_SYSTEM_PROCESSOR x86_64)
+
+# specify the cross compiler
+set(CMAKE_C_COMPILER x86_64-w64-mingw32-gcc)
+set(CMAKE_CXX_COMPILER x86_64-w64-mingw32-g++)
+set(CMAKE_RC_COMPILER x86_64-w64-mingw32-windres)
+
+# where is the target environment
+set(CMAKE_FIND_ROOT_PATH /usr/x86_64-w64-mingw32)
+
+# search for programs in the build host directories
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+# for libraries and headers in the target directories
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)