CI: Add instance for Clang on macOS in configure workflow.
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
index 329a4da..df5ca5c 100644
--- a/.github/workflows/configure.yml
+++ b/.github/workflows/configure.yml
@@ -78,6 +78,11 @@
             compiler: gcc-9
             configure-args: --warn
 
+          - name: macOS Clang
+            os: macos-latest
+            compiler: clang
+            configure-args: --warn
+
     steps:
     - name: Checkout repository
       uses: actions/checkout@v3