Sysctl has been deprecated for a while and in newer versions of glibc it
is removed which leads to a compilation error on modern Linux. This is
fixed in SVN trunk of irrlich so I copied the include guards from there.
In this case sysctl headers are only loaded for OS X where they are also
actually used.
Since libjpeg, libpng, zlib and bzlib are provided as dependencies
through vcpkg, we disable building them in irrlicht. This fixes a
runtime error steming from a version mismatch between the libjpeg
versions provided by irrlicht and vcpkg respectively
