Added the Clang compiler support.

The -Werror is commented out to not break builds on Linux.
diff --git a/auto/cc/conf b/auto/cc/conf
index c9118f2..8027b98 100644
--- a/auto/cc/conf
+++ b/auto/cc/conf
@@ -56,6 +56,12 @@
             . auto/cc/gcc
         ;;
 
+        clang)
+            # Clang C compiler
+
+            . auto/cc/clang
+        ;;
+
         icc)
             # Intel C++ compiler 7.1, 8.0, 8.1