github: workflow: Add build test

XC8 license seems to allow us use the "Free" version on GitHub
Actions.  The support team also says:

    Do you see any issues installing it on your machine?  Try
    downloading the free version and let us now if you have any
    issues.  If you need to avail the PRO features the only license
    combination that supports CI is the SW006021-2N and the
    SW006021-VM.
    https://www.microchipdirect.com/product/search/all/SW006021-2N

So, here we have it.

Signed-off-by: Yasushi SHOJI <[email protected]>

.github/workflows/build.yaml

@@ -0,0 +1,41 @@
+name: Build
+on: [push, pull_request]
+env:
+  XC8_URL: https://ww1.microchip.com/downloads/aemDocuments/documents/DEV/ProductDocuments/SoftwareTools
+  XC8_INSTALL_DIR: microchip/xc8
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        xc8-version:
+          - 'v2.46'
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Cache XC8
+        uses: actions/cache@v4
+        id: cache-xc8
+        with:
+          path: ${{ env.XC8_INSTALL_DIR }}
+          key: xc8-${{ matrix.xc8-version }}-full-install-linux-x64-installer.run
+
+      - name: Download XC8
+        if: steps.cache-xc8.outputs.cache-hit != 'true'
+        run: |
+          wget $XC8_URL/xc8-${{ matrix.xc8-version }}-full-install-linux-x64-installer.run
+          chmod +x *.run
+
+      - name: Install XC8
+        if: steps.cache-xc8.outputs.cache-hit != 'true'
+        run: |
+          fakeroot ./*.run --mode unattended --netservername localhost --LicenseType FreeMode --prefix $GITHUB_WORKSPACE/$XC8_INSTALL_DIR
+
+      - name: Build
+        run: |
+          export PATH=$GITHUB_WORKSPACE/$XC8_INSTALL_DIR/bin:$PATH
+          make