Add clang as a compiler for CI

Build with gcc and clang, to catch problems with either.

.github/workflows/c-cpp.yml

@@ -2,20 +2,35 @@ name: C/C++ CI
 
 on:
   push:
-    branches: [ "master", "ci" ]
+    branches: [ "master", "clang-ci" ]
   pull_request:
-    branches: [ "master", "ci" ]
+    branches: [ "master", "clang-ci" ]
 
 jobs:
-  build:
+  build-gcc:
 
     runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v3
     - name: configure
-      run: cmake -DCMAKE_BUILD_TYPE=Release
+      run: |
+        sudo apt install g++
+        cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=g++
     - name: make
-      run: make
+      run: make VERBOSE=1
+    - name: make check
+      run: make check CTEST_OUTPUT_ON_FAILURE=TRUE
+
+  build-clang:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: configure
+      run: |
+        sudo apt install clang
+        cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang
+    - name: make
+      run: make VERBOSE=1
     - name: make check
       run: make check CTEST_OUTPUT_ON_FAILURE=TRUE