#317 Compile with musl c library

[Felix Janda]

musl is an alternative c library for linux systems. (http://musl-libc.org)

Currently, Irrlicht cannot be compiled with musl libc because COSOperator.cpp tries to include <sys sysctl.h="">, which musl lacks. On the other hand, the code uses only sysctl() for Mac systems.

So the attached patch make the <sys sysctl.h=""> to be included only for Mac.

Also, as a cleanup, it makes the <sys types.h=""> include unconditional for UNIX. This header is also present on Solaris systems.

[Michael Zeilfelder]

Thanks. I've applied it svn trunk r5349.
I'll trust you on the Solaris thing :-) (Our only Solaris developer is no longer active, so support might get dropped anyway as we no longer have anyone doing test-builds for that).