Grep for clang warnings when testing compiler flag support

There is no way to make clang's "argument unused" warning fatal.

Ideally the search string would also include 'clang: ' but this output might
depend on clang's argv[0].

scripts/wrap-compiler-for-flag-check

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+# There is no way to make clang's "argument unused" warning fatal.  So when
+# configure checks for supported flags, it runs $CC, $CXX, $LD via this
+# wrapper.
+#
+# Ideally the search string would also include 'clang: ' but this output might
+# depend on clang's argv[0].
+
+if out=`"$@" 2>&1`; then
+  echo "$out"
+  if echo "$out" | grep 'warning: argument unused' >/dev/null; then
+    echo "$0: found clang warning"
+    exit 1
+  else
+    exit 0
+  fi
+else
+  code=$?
+  echo "$out"
+  exit $code
+fi