Do not use -Werror on gcc versions under 7

zcutil/build.sh

@@ -104,10 +104,10 @@ ld -v
 HOST="$HOST" BUILD="$BUILD" NO_PROTON="$PROTON_ARG" "$MAKE" "$@" -C ./depends/ V=1
 ./autogen.sh
 
-# 0x03 2020-01-03, do not use '-Wno-builtin-declaration-mismatch on GCC <v7
+# 0x03 2020-01-03, do not use '-Wno-builtin-declaration-mismatch or -Werror on GCC <v7
 if [ "$(gcc --version|head -1 | awk '{print $4}' | cut -d"." -f1)" -lt 7 ]; then
 	# version <7
-	CONFIG_SITE="$PWD/depends/$HOST/share/config.site" ./configure "$HARDENING_ARG" "$LCOV_ARG" "$TEST_ARG" "$MINING_ARG" "$PROTON_ARG" $CONFIGURE_FLAGS CPPFLAGS='-g -Werror' CXXFLAGS='-g'
+	CONFIG_SITE="$PWD/depends/$HOST/share/config.site" ./configure "$HARDENING_ARG" "$LCOV_ARG" "$TEST_ARG" "$MINING_ARG" "$PROTON_ARG" $CONFIGURE_FLAGS CPPFLAGS='-g' CXXFLAGS='-g'
 else
 	# version 7 and up
 	CONFIG_SITE="$PWD/depends/$HOST/share/config.site" ./configure "$HARDENING_ARG" "$LCOV_ARG" "$TEST_ARG" "$MINING_ARG" "$PROTON_ARG" $CONFIGURE_FLAGS CPPFLAGS='-g -Wno-builtin-declaration-mismatch -Werror' CXXFLAGS='-g'