Travis: Run clang-format static check

Commits or PRs that do not respect the clang-format style that we
enforce will fail the test.

Adding ubuntu-toolchain-r-test as its libstdc++ is necessary to install
libllvm-3.9.

misc/travis/clang-format.sh

@@ -0,0 +1,40 @@
+#!/bin/sh
+
+CLANG_FORMAT=clang-format-3.9
+
+if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
+    # Check the whole commit range against $TRAVIS_BRANCH, the base merge branch
+    # We could use $TRAVIS_COMMIT_RANGE but it doesn't play well with force pushes
+    RANGE="$(git rev-parse $TRAVIS_BRANCH) HEAD"
+else
+    # Test only the last commit
+    RANGE=HEAD
+fi
+
+FILES=$(git diff-tree --no-commit-id --name-only -r $RANGE | grep -v thirdparty/ | grep -e "\.cpp$" -e "\.h$" -e "\.inc$")
+echo "Checking files:\n$FILES"
+
+# create a random filename to store our generated patch
+prefix="static-check-clang-format"
+suffix="$(date +%s)"
+patch="/tmp/$prefix-$suffix.patch"
+
+for file in $FILES; do
+    "$CLANG_FORMAT" -style=file "$file" | \
+        diff -u "$file" - | \
+        sed -e "1s|--- |--- a/|" -e "2s|+++ -|+++ b/$file|" >> "$patch"
+done
+
+# if no patch has been generated all is ok, clean up the file stub and exit
+if [ ! -s "$patch" ] ; then
+    printf "Files in this commit comply with the clang-format rules.\n"
+    rm -f "$patch"
+    exit 0
+fi
+
+# a patch has been created, notify the user and exit
+printf "\n*** The following differences were found between the code to commit "
+printf "and the clang-format rules:\n\n"
+cat "$patch"
+printf "\n*** Aborting, please fix your commit(s) with 'git commit --amend' or 'git rebase -i <hash>'\n"
+exit 1