Better way of enabling Developer Toolset 7 for Jenkins' linux_gcc_release_64

Enabling it in the relevant distro-configs/Jenkins/* file is more intuitive than
enabling it in autogen.sh (and avoids issues like
d057e61c "Restrict Developer Toolset to
Config=linux_gcc_release_64"), and will also be used for other Jenkins jobs like
<https://gerrit.libreoffice.org/#/c/63982/> "Enabling Developer Toolset 7 for
Jenkins' lo_tb_master_linux_dbg".

Change-Id: If633044a90c35a12a73d60335839af0a106aa20f
Reviewed-on: https://gerrit.libreoffice.org/63989
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

autogen.sh

@@ -151,13 +151,6 @@ if (defined $ENV{LODE_HOME})
         $ENV{PATH}="$ENV{LODE_HOME}/opt/bin:$ENV{PATH}";
         print STDERR "add LODE_HOME/opt/bin in PATH\n";
     }
-    # For the Config=linux_gcc_release_64 Jenkins jobs that build on CentOS against Developer
-    # Toolset 7:
-    if ($ENV{Config} eq 'linux_gcc_release_64' && -d '/opt/rh/devtoolset-7/root/usr/bin')
-    {
-        $ENV{PATH}="/opt/rh/devtoolset-7/root/usr/bin:$ENV{PATH}";
-        print STDERR "added /opt/rh/devtoolset-7/root/usr/bin to PATH\n";
-    }
 }
 
 my $aclocal_flags = $ENV{ACLOCAL_FLAGS};

distro-configs/Jenkins/linux_gcc_release_64

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