CI: Update for meson build

We now include a Meson build that uses
Clang + lld as the build system to catch bugs and warnings
that our traditional autotools + GCC builds might miss.

This commit changes the autotools build to be more like the
meson build path: We no longer build a docker image, rather
directly executing the fvwm3-build container as a github
action.

The Go version in fvwm3-tools has been updated to enable use
with meson and eliminate the need for a complex envvar setup
and invocation, however older golang is still supported (>=1.14);
we're just relying on users to tell us if something is broken.

.github/workflows/ccpp.yml

@@ -10,13 +10,32 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    container:
+      image: fvwmorg/fvwm3-build:latest
+      env:
+        GO111MODULE: "on"
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+      - name: set git safe directory
+        run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+      - name: Autotools configure
+        run: ./autogen.sh && ./configure --enable-mandoc --enable-golang
+      - name: Autotools build
+        run: make -j
 
+  build-meson:
+    runs-on: ubuntu-latest
+    container: fvwmorg/fvwm3-build:latest
     steps:
-    - uses: actions/checkout@v2
-    - name: Pulling docker image
-      run: docker pull fvwmorg/fvwm3-build:latest
-    - name: Build Package
-      run: 'docker build -t fvwm3 .'
+      - name: checkout
+        uses: actions/checkout@v4
+      - name: set git safe directory
+        run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+      - name: configure meson build
+        run:  CC=clang CC_LD=lld meson setup builddir -Dhtmldoc=true -Dmandoc=true -Dauto_features=enabled
+      - name: run build
+        run: ninja -C builddir
 
   notification:
     runs-on: ubuntu-20.04