Install internal ivykis headers when necessary

When building with an internal ivykis, and it gets linked into libsyslog-ng, install the headers to `${pkgincludedir}/ivykis`, adjust the `CFLAGS` in the pkg-config control file accordingly, and note the fact with an `ivykis=internal` entry in the same file.

This makes it possible to not only detect a syslog-ng with an embedded ivykis, but to build against such a thing, too. This fixes #124.