[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!)

Author: ISSOtm

.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-mingw64.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-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)