Add warning that enabling compdb disables ccache.

Clang uses an additional compiler option to generate the data files
that are compiled into compdb.  Ccache doesn't understand this extra
argument, so it falls back to a full compile of every file.

mythtv/configure

@@ -6479,6 +6479,9 @@ case "$cc_type" in
         if test $target_os = "darwin"; then
             disable enforce_nullptr
         fi
+        if enabled compdb; then
+            warn "Enabling clang's compdb support will disable ccache."
+        fi
         ;;
 esac