build: add --enable-werror option

This turns some compiler warnings into errors. Useful for c-i.

configure.ac

@@ -158,6 +158,13 @@ AC_ARG_ENABLE([debug],
     [enable_debug=$enableval],
     [enable_debug=no])
 
+# Turn warnings into errors
+AC_ARG_ENABLE([werror],
+    [AS_HELP_STRING([--enable-werror],
+                    [Treat certain compiler warnings as errors (default is no)])],
+    [enable_werror=$enableval],
+    [enable_werror=no])
+
 AC_LANG_PUSH([C++])
 AX_CHECK_COMPILE_FLAG([-Werror],[CXXFLAG_WERROR="-Werror"],[CXXFLAG_WERROR=""])
 
@@ -172,6 +179,14 @@ if test "x$enable_debug" = xyes; then
     fi
 fi
 
+ERROR_CXXFLAGS=
+if test "x$enable_werror" = "xyes"; then
+  if test "x$CXXFLAG_WERROR" = "x"; then
+    AC_MSG_ERROR("enable-werror set but -Werror is not usable")
+  fi
+  AX_CHECK_COMPILE_FLAG([-Werror=vla],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=vla"],,[[$CXXFLAG_WERROR]])
+fi
+
 if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wall],[CXXFLAGS="$CXXFLAGS -Wall"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wextra],[CXXFLAGS="$CXXFLAGS -Wextra"],,[[$CXXFLAG_WERROR]])
@@ -816,6 +831,7 @@ AC_SUBST(BITCOIN_CLI_NAME)
 AC_SUBST(BITCOIN_TX_NAME)
 
 AC_SUBST(RELDFLAGS)
+AC_SUBST(ERROR_CXXFLAGS)
 AC_SUBST(HARDENED_CXXFLAGS)
 AC_SUBST(HARDENED_CPPFLAGS)
 AC_SUBST(HARDENED_LDFLAGS)
@@ -884,6 +900,7 @@ echo "  with wallet   = $enable_wallet"
 echo "  with zmq      = $use_zmq"
 echo "  with test     = $use_tests"
 echo "  debug enabled = $enable_debug"
+echo "  werror        = $enable_werror"
 echo 
 echo "  target os     = $TARGET_OS"
 echo "  build os      = $BUILD_OS"

src/Makefile.am

@@ -1,7 +1,7 @@
 DIST_SUBDIRS = secp256k1 univalue
 
 AM_LDFLAGS = $(PTHREAD_CFLAGS) $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS)
-AM_CXXFLAGS = $(HARDENED_CXXFLAGS)
+AM_CXXFLAGS = $(HARDENED_CXXFLAGS) $(ERROR_CXXFLAGS)
 AM_CPPFLAGS = $(HARDENED_CPPFLAGS)
 
 if EMBEDDED_LEVELDB