Add Windows build support via MSYS2 and fix platform-specific code

Guard X11 display crash handler with __linux__, add Windows GlobalMemoryStatusEx
path in memory_monitor, guard warden cache paths with APPDATA on Windows, and
make pkg-config optional in CMakeLists with a find_library fallback. Add Windows
x86-64 CI job using MSYS2 MINGW64 to the build workflow.

.github/workflows/build.yml

@@ -64,3 +64,47 @@ jobs:
           name: wowee-${{ matrix.arch }}
           path: build/bin/
           if-no-files-found: error
+
+  build-windows:
+    name: Build (windows-x86-64)
+    runs-on: windows-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up MSYS2
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: MINGW64
+          update: false
+          install: >-
+            mingw-w64-x86_64-cmake
+            mingw-w64-x86_64-gcc
+            mingw-w64-x86_64-pkgconf
+            mingw-w64-x86_64-SDL2
+            mingw-w64-x86_64-glew
+            mingw-w64-x86_64-glm
+            mingw-w64-x86_64-openssl
+            mingw-w64-x86_64-zlib
+            mingw-w64-x86_64-ffmpeg
+            mingw-w64-x86_64-unicorn
+            git
+
+      - name: Clone ImGui
+        shell: msys2 {0}
+        run: git clone --depth 1 https://github.com/ocornut/imgui.git extern/imgui
+
+      - name: Configure
+        shell: msys2 {0}
+        run: cmake -S . -B build -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release
+
+      - name: Build
+        shell: msys2 {0}
+        run: cmake --build build --parallel $(nproc)
+
+      - name: Upload artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: wowee-windows-x86-64
+          path: build/bin/
+          if-no-files-found: error