[RFC PATCH 4/6] travis.yml: Fix 'Clang (main-softmmu)' job on Ubuntu Bionic

[Wainer dos Santos Moschetta <wainersm@redhat.com>]

On Ubuntu Bionic environment the build with Clang's sanitizer
fails with error:

```
LINK    tests/qemu-iotests/socket_scm_helper

tests/qemu-iotests/socket_scm_helper.o: In function `send_fd':

/home/travis/build/wainersm/qemu/tests/qemu-iotests/socket_scm_helper.c:53: undefined reference to `__ubsan_handle_type_mismatch_v1'
```

In order to solve that problem it was needed to explicitly pass
'--cxx=clang++' to the configure.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
---
 .travis.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.travis.yml b/.travis.yml
index 4089e9672a..59b9eb20b7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -215,7 +215,7 @@ matrix:
       compiler: clang
       before_script:
         - mkdir -p ${BUILD_DIR} && cd ${BUILD_DIR}
-        - ${SRC_DIR}/configure ${CONFIG} --extra-cflags="-fsanitize=undefined -Werror" || { cat config.log && exit 1; }
+        - ${SRC_DIR}/configure ${CONFIG} --cc=clang --cxx=clang++ --extra-cflags="-fsanitize=undefined -Werror" || { cat config.log && exit 1; }
 
 
     - name: "Clang (other-softmmu)"
-- 
2.24.1