gitlab-ci: Install most packages from Debian buster

We now use the C frontend of GCC 8 instead of 6 (required tweaking the
before_script for the clang job). We cannot use the C++ frontend of GCC
7 or newer yet, because upstream GCC 7 changed some C++ name mangling
stuff in backwards incompatible ways, and LLVM < 6.0 packages aren't
available in buster.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 3a53fae..719fcb0 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -66,12 +66,13 @@
   artifacts:
     when: on_failure
     untracked: true
+  variables:
+    CCACHE_COMPILERCHECK: "content"
   # Use ccache transparently, and print stats before/after
   before_script:
     - export PATH="/usr/lib/ccache:$PATH"
     - export CCACHE_BASEDIR="$PWD"
     - export CCACHE_DIR="$PWD/ccache"
-    - export CCACHE_COMPILERCHECK=content
     - ccache --zero-stats || true
     - ccache --show-stats || true
   after_script:
@@ -148,6 +149,11 @@
     GALLIUM_DRIVERS: "auto"
     CC: "ccache clang-7"
     CXX: "ccache clang++-7"
+  before_script:
+    - export CCACHE_BASEDIR="$PWD" CCACHE_DIR="$PWD/ccache"
+    - ccache --zero-stats --show-stats || true
+     # clang++ breaks if it picks up the GCC 8 directory without libstdc++.so
+    - apt-get remove -y libgcc-8-dev
 
 meson-vulkan:
   extends: .meson-build