- Moved ``<sys/capability.h>`` include inside `__linux__` guard,
because all uses are similarly guarded.
- <sys/capability.h> is part of an optional devel package, at least
in fedora. CMake now explicitly checks that libcap devel files
are available.
- Added libcap to the list of install packages in .travis.yml, to
make the dependency explicit. travis-ci installs the package by
default, which is why this hasn't surfaced previously.
