curl-compilers.m4: enable -Wshift-sign-overflow for clang

clang 2.9+ supports -Wshift-sign-overflow, which warns about undefined
behavior on signed left shifts when shifting by too many places.

Ref: https://github.com/curl/curl/issues/1516
Closes https://github.com/curl/curl/pull/1517

m4/curl-compilers.m4

@@ -881,6 +881,11 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           if test "$compiler_num" -ge "101"; then
             tmp_CFLAGS="$tmp_CFLAGS -Wunused"
           fi
+          #
+          dnl Only clang 2.9 or later
+          if test "$compiler_num" -ge "209"; then
+            tmp_CFLAGS="$tmp_CFLAGS -Wshift-sign-overflow"
+          fi
         fi
         ;;
         #