CI: extern "C" check

Add a script that checks headers for `extern "C"` declaration.
Run this script in Travis.
You could use
```bash
if [ -z "${BRANCH}" ]; then
    git ls-tree -r --full-tree --name-only HEAD | grep -E '\.h$'
else
    git diff --diff-filter=ACMR --name-only ${BRANCH} | grep -E '\.h$'
fi | while read FILE; do
    …
done
```