Jenkins' lo_tb_master_linux needs --disable-werror too

...similar to d3f2c61e "Make Jenkins
linux_gcc_release_64 pick up Developer Toolset 7":

> In file included from /opt/rh/devtoolset-7/root/usr/include/c++/7/vector:69:0,
>                  from /home/tdf/lode/jenkins/workspace/lo_tb_master_linux/jvmfwk/plugins/sunmajor/pluginlib/sunjavaplugin.cxx:39:
> /opt/rh/devtoolset-7/root/usr/include/c++/7/bits/vector.tcc: In function ‘javaPluginError jfw_plugin_startJavaVirtualMachine(const JavaInfo*, const JavaVMOption*, sal_Int32, JavaVM**, JNIEnv**)’:
> /opt/rh/devtoolset-7/root/usr/include/c++/7/bits/vector.tcc:407:15: error: variable ‘__new_finish’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
>        pointer __new_finish(__new_start);
>                ^~~~~~~~~~~~
> cc1plus: all warnings being treated as errors

(<https://ci.libreoffice.org/job/lo_tb_master_linux/32166/>).

Change-Id: Ifae8705bfafe4caed21713909ff6eeee158f1361

distro-configs/Jenkins/Linux_rel_master.conf

+--disable-werror
 --enable-kde4
 CC=/opt/rh/devtoolset-7/root/usr/bin/gcc
 CXX=/opt/rh/devtoolset-7/root/usr/bin/g++