ART: More warnings

Enable -Wno-conversion-null, -Wredundant-decls and -Wshadow in general,
and -Wunused-but-set-parameter for GCC builds.

Change-Id: I81bbdd762213444673c65d85edae594a523836e5
diff --git a/sigchainlib/sigchain_dummy.cc b/sigchainlib/sigchain_dummy.cc
index 0d62a7a..76779ab 100644
--- a/sigchainlib/sigchain_dummy.cc
+++ b/sigchainlib/sigchain_dummy.cc
@@ -41,6 +41,9 @@
   va_end(ap);
 }
 
+namespace art {
+
+
 extern "C" void ClaimSignalChain(int signal ATTRIBUTE_UNUSED,
                                  struct sigaction* oldaction ATTRIBUTE_UNUSED) {
   log("ClaimSignalChain is not exported by the main executable.");
@@ -69,3 +72,5 @@
   log("EnsureFrontOfChain is not exported by the main executable.");
   abort();
 }
+
+}  // namespace art