Enable -Wcovered-switch-default again, but add -Werror to the checks to make
sure we don't use it with compilers that don't support it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151665 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/projects/sample/autoconf/configure.ac b/projects/sample/autoconf/configure.ac
index 1caff51..a1110ec 100644
--- a/projects/sample/autoconf/configure.ac
+++ b/projects/sample/autoconf/configure.ac
@@ -1019,7 +1019,8 @@
 AC_MSG_CHECKING([optional compiler flags])
 CXX_FLAG_CHECK(NO_VARIADIC_MACROS, [-Wno-variadic-macros])
 CXX_FLAG_CHECK(NO_MISSING_FIELD_INITIALIZERS, [-Wno-missing-field-initializers])
-AC_MSG_RESULT([$NO_VARIADIC_MACROS $NO_MISSING_FIELD_INITIALIZERS])
+CXX_FLAG_CHECK(COVERED_SWITCH_DEFAULT, [-Wcovered-switch-default])
+AC_MSG_RESULT([$NO_VARIADIC_MACROS $NO_MISSING_FIELD_INITIALIZERS $COVERED_SWITCH_DEFAULT])
 
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 dnl===