gitlab-ci: Use LLVM 3.4 from Debian jessie for scons-llvm job

This gets us closer to the officially supported minimum version of LLVM,
which is 3.3.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>

.gitlab-ci.yml

@@ -315,7 +315,9 @@ scons-llvm:
   variables:
     SCONS_TARGET: "llvm=1"
     SCONS_CHECK_COMMAND: "scons llvm=1 check"
-    LLVM_VERSION: "3.9"
+    LLVM_VERSION: "3.4"
+    # LLVM 3.4 packages were built with an old libstdc++ ABI
+    CXX: "g++ -D_GLIBCXX_USE_CXX11_ABI=0"
 
 scons-swr:
   extends: .scons-build

.gitlab-ci/debian-install.sh

@@ -19,9 +19,11 @@ add-apt-repository "deb https://apt.llvm.org/stretch/ llvm-toolchain-stretch-8 m
 
 sed -i -e 's/http:\/\/deb/https:\/\/deb/g' /etc/apt/sources.list
 echo 'deb https://deb.debian.org/debian stretch-backports main' >/etc/apt/sources.list.d/backports.list
+echo 'deb https://deb.debian.org/debian jessie main' >/etc/apt/sources.list.d/jessie.list
 
 apt-get update
 apt-get install -y -t stretch-backports \
+      llvm-3.4-dev \
       llvm-3.9-dev \
       libclang-3.9-dev \
       llvm-5.0-dev \