Check for C99 features which may not be supported

Continue on error because it is known that this fails right now.
Once fixed, we'll turn it off.

.github/workflows/check_misc.yml

@@ -52,6 +52,14 @@ jobs:
           exit $fail
         working-directory: include
 
+      # This should be able to be removed once old platforms are dropped.
+      - id: c99-check
+        name: Check for C99 features which may not be supported
+        run: |
+          # VC2013: z modifier for size_t is not supported.
+          grep -r -n --include='*.[chyS]' '%[0-9]*z[idu]' -- . && exit 1 || :
+        continue-on-error: true # Just show in the annotations
+
       - id: gems
         run: true
         if: ${{ github.ref == 'refs/heads/master' }}