...as discussed at
<https://lists.freedesktop.org/archives/libreoffice/2018-November/081423.html>
"Re: Compiler baselines".

It doesn't look exactly right to enable the Developer Toolset from autogen.sh.
But the alternative would be to "hide" that in
<https://ci.libreoffice.org/job/gerrit_linux_gcc_release/configure>, which would
probably not be helpful when developers try to track down why a certain Jenkins
build behaves the way it does.  So pragmatically stick it in autogen.sh.  (Also,
it puts Developer Toolset on the PATH whenever it is found on a system using
LODE_HOME, not just for the specific Config=linux_gcc_release_64 case.  Lets see
how that works out in practice.)

However, it turns out that the Developer Toolset 7's GCC 7.3.1 with
--enable-werror (that is implicitly enabled for LODE-driven builds in
configure.ac) and (implicit) --enable-optimized produces many false warnings
(i.e., errors), see below for a sample.  (Actually, my experience is that
contemporary GCC hardly ever work with -Werror in optimized builds, due to
analysis being done on already optimized code; it surprised me to find out that
the Jnekins linux_gcc_release_64 builds were apparently successfully done with
--enable-werror with GCC 4.8.5.)  So explicitly --disable-werror for these
builds.  (Which means that <https://gerrit.libreoffice.org/plugins/gitiles/lode/
+/b82e0a9d26ef4c81046c053ff831dccfc84c56be%5E!> "For linux_gcc_release_64, don't
let ccache strip comments" could probably be reverted again if it has negative
impact on Jenkins' performance.)

Some of the false warnings encountered:

> [CXX] jvmfwk/plugins/sunmajor/pluginlib/sunjavaplugin.cxx
> In file included from /opt/rh/devtoolset-7/root/usr/include/c++/7/vector:69:0,
>                  from /home/tdf/sberg/core/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

> [CXX] libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx: In function ‘gboolean gtv_calc_header_bar_draw(GtkWidget*, cairo_t*)’:
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:89:117: error: ‘aRectangle._cairo_rectangle_int::height’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      cairo_move_to(pCairo, rRectangle.x + rRectangle.width / 2 - extents.width / 2, rRectangle.y + rRectangle.height / 2 + extents.height / 2);
>                                                                                                    ~~~~~~~~~~~~~~~~~~^~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:102:22: note: ‘aRectangle._cairo_rectangle_int::height’ was declared here
>          GdkRectangle aRectangle;
>                       ^~~~~~~~~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:89:59: error: ‘aRectangle._cairo_rectangle_int::width’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      cairo_move_to(pCairo, rRectangle.x + rRectangle.width / 2 - extents.width / 2, rRectangle.y + rRectangle.height / 2 + extents.height / 2);
>                                           ~~~~~~~~~~~~~~~~~^~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:102:22: note: ‘aRectangle._cairo_rectangle_int::width’ was declared here
>          GdkRectangle aRectangle;
>                       ^~~~~~~~~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:89:97: error: ‘aRectangle._cairo_rectangle_int::y’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      cairo_move_to(pCairo, rRectangle.x + rRectangle.width / 2 - extents.width / 2, rRectangle.y + rRectangle.height / 2 + extents.height / 2);
>                                                                                     ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:102:22: note: ‘aRectangle._cairo_rectangle_int::y’ was declared here
>          GdkRectangle aRectangle;
>                       ^~~~~~~~~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:89:40: error: ‘aRectangle._cairo_rectangle_int::x’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      cairo_move_to(pCairo, rRectangle.x + rRectangle.width / 2 - extents.width / 2, rRectangle.y + rRectangle.height / 2 + extents.height / 2);
>                            ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
> /home/tdf/sberg/core/libreofficekit/qa/gtktiledviewer/gtv-calc-header-bar.cxx:102:22: note: ‘aRectangle._cairo_rectangle_int::x’ was declared here
>          GdkRectangle aRectangle;
>                       ^~~~~~~~~~
> cc1plus: all warnings being treated as errors

> [CXX] svl/source/misc/lockfilecommon.cxx
> /home/tdf/sberg/core/svl/source/misc/lockfilecommon.cxx: In static member function ‘static rtl::OUString svt::LockFileCommon::GetCurrentLocalTime()’:
> /home/tdf/sberg/core/svl/source/misc/lockfilecommon.cxx:190:10: error: ‘%02d’ directive writing between 2 and 5 bytes into a region of size between 1 and 9 [-Werror=format-overflow=]
>  OUString LockFileCommon::GetCurrentLocalTime()
>           ^~~~~~~~~~~~~~
> /home/tdf/sberg/core/svl/source/misc/lockfilecommon.cxx:190:10: note: directive argument in the range [0, 65535]
> /home/tdf/sberg/core/svl/source/misc/lockfilecommon.cxx:190:10: note: directive argument in the range [0, 65535]
> /home/tdf/sberg/core/svl/source/misc/lockfilecommon.cxx:204:24: note: ‘sprintf’ output between 17 and 31 bytes into a destination of size 20
>                  sprintf( pDateTime, "%02d.%02d.%4d %02d:%02d", aDateTime.Day, aDateTime.Month, aDateTime.Year, aDateTime.Hours, aDateTime.Minutes );
>                  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> cc1plus: all warnings being treated as errors

Change-Id: I3a851b7591274a8cf8b4729ae036afeb8e82eedc
Reviewed-on: https://gerrit.libreoffice.org/63884
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>