travis: add Trusty with clang stable build

Although we've reduced the matrix to avoid repeating clang builds we can
still add an additional clang build to use the latest stable version of
clang which will typically be available on current distros.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -103,6 +103,26 @@ matrix:
         - sudo apt-get build-dep -qq qemu
         - wget -O - http://people.linaro.org/~alex.bennee/qemu-submodule-git-seed.tar.xz | tar -xvJ
         - git submodule update --init --recursive
+    # Trusty build with latest stable clang
+    - env: CONFIG=""
+      sudo: required
+      addons:
+      dist: trusty
+      language: generic
+      compiler: none
+      env:
+        - COMPILER_NAME=clang CXX=clang++-3.9 CC=clang-3.9
+        - CONFIG="--cc=clang-3.9 --cxx=clang++-3.9"
+      before_install:
+        - wget -nv -O - http://llvm.org/apt/llvm-snapshot.gpg.key | sudo apt-key add -
+        - sudo apt-add-repository -y 'deb http://llvm.org/apt/trusty llvm-toolchain-trusty-3.9 main'
+        - sudo apt-get update -qq
+        - sudo apt-get install -qq -y clang-3.9
+        - sudo apt-get build-dep -qq qemu
+        - wget -O - http://people.linaro.org/~alex.bennee/qemu-submodule-git-seed.tar.xz | tar -xvJ
+        - git submodule update --init --recursive
+      before_script:
+        - ./configure ${CONFIG} || cat config.log
     # Using newer GCC with sanitizers
     - addons:
         apt: