Add DOS/DJGPP cross-compilation support and CI for Linux + macOS

Add a DJGPP CMake toolchain file and DOS compatibility layer (err.h,
fnmatch, getprogname/setprogname) so UC2 builds as a native DOS
executable via cross-compilation from Linux.  The toolchain works
around a baked-in /usr/include in the DJGPP GCC binary by using -I
instead of -isystem to ensure DJGPP headers take precedence.

Add GitHub Actions CI workflow that builds and smoke-tests on both
ubuntu-latest and macos-latest.

.github/workflows/build.yml

@@ -0,0 +1,23 @@
+name: Build
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Configure
+        run: cmake -B build -DCMAKE_BUILD_TYPE=Release
+      - name: Build
+        run: cmake --build build
+      - name: Smoke test
+        run: ./build/cli/uc2 -h