build.c: use C macros + string literal concatenation for ASAN flags (#1235)

* Initial plan

* Add ASAN binary builds targeting binaries-asan branch

Co-authored-by: uNetworkingAB <110806833+uNetworkingAB@users.noreply.github.com>

* Refactor ASAN flags in build.c to use C macros and string concatenation

Co-authored-by: uNetworkingAB <110806833+uNetworkingAB@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: uNetworkingAB <110806833+uNetworkingAB@users.noreply.github.com>

.github/workflows/build-asan.yml

@@ -0,0 +1,37 @@
+name: Build ASAN
+on:
+  push:
+    branches: [ master ]
+jobs:
+  build-asan:
+    runs-on: ${{ matrix.os }}
+    permissions:
+      contents: read
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [macos-15, ubuntu-24.04, ubuntu-24.04-arm]
+    steps:
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 25
+      - name: Update ASAN binaries
+        run: |
+          git clone --recursive https://github.com/uNetworking/uWebSockets.js.git
+          cd uWebSockets.js
+          cc build.c -DWITH_ASAN && ./a.out
+          ls dist
+          # On first run, binaries-asan doesn't exist yet; seed it from binaries so git history is preserved
+          git fetch origin binaries-asan:binaries-asan 2>/dev/null || git fetch origin binaries:binaries-asan
+          git checkout binaries-asan
+          cp dist/*.node .
+          cp dist/*.js .
+          git rev-parse master > source_commit
+          ls
+          git checkout master docs/index.d.ts && mv docs/index.d.ts .
+          git status
+          git config --global user.email "alexhultman@gmail.com"
+          git config --global user.name "Alex Hultman"
+          git commit -a -m "[GitHub Actions] Updated ${{ matrix.os }} ASAN binaries" || true
+          git push "https://unetworkingab:${{ secrets.SECRET }}@github.com/uNetworking/uWebSockets.js" binaries-asan