travis: display config.log when configure fails

When configure fails in CI systems we must be able to see the contents
of the config.log file to diagnose the root cause.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
[AJB: used Eric's suggested {} form]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -69,7 +69,7 @@ before_install:
   - 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}
+  - ./configure ${CONFIG} || { cat config.log && exit 1; }
 script:
   - make ${MAKEFLAGS} && ${TEST_CMD}
 matrix:
@@ -151,4 +151,4 @@ matrix:
         - CONFIG="--cc=gcc-7 --cxx=g++-7 --disable-pie --disable-linux-user"
         - TEST_CMD=""
       before_script:
-        - ./configure ${CONFIG} --extra-cflags="-g3 -O0 -fsanitize=thread -fuse-ld=gold" || cat config.log
+        - ./configure ${CONFIG} --extra-cflags="-g3 -O0 -fsanitize=thread -fuse-ld=gold" || { cat config.log && exit 1; }