.travis.yml: enable OSX builds

Travis has support for OSX builds. Making the setup work cleanly
involves a little hacking about with the .travis.yml file but rather
than make it too messy I've pushed all the "brew" install stuff into a
support script called ./scripts/macosx-brew.sh.

Currently only the default ./configure ${CONFIG} is built as I'm not
sure what extra coverage would come from the other build stanzas.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Peter Maydell <peter.maydell@linaro.org>

.travis.yml

@@ -53,6 +53,8 @@ git:
   # we want to do this ourselves
   submodules: false
 before_install:
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update ; fi
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew install libffi gettext glib pixman ; fi
   - wget -O - http://people.linaro.org/~alex.bennee/qemu-submodule-git-seed.tar.xz | tar -xvJ
   - git submodule update --init --recursive
 before_script:
@@ -83,3 +85,6 @@ matrix:
     - env: CONFIG="--with-coroutine=gthread"
            TEST_CMD=""
       compiler: gcc
+    - env: CONFIG=""
+      os: osx
+      compiler: clang