gitlab-ci: autotools needs to be told which llvm version to use

Fixes: 45d58cd915 "gitlab-ci: only build the default (=latest) and oldest llvm versions"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>

.gitlab-ci.yml

@@ -423,6 +423,7 @@ make-gallium-st-clover-llvm:
       --disable-gallium-osmesa
     LIBUNWIND_FLAGS: --enable-libunwind
     GALLIUM_DRIVERS: "r600,radeonsi"
+    LLVM_VERSION: "7.0"
 
 make-gallium-st-clover-llvm-39:
   extends: make-gallium-st-clover-llvm