freebsd: features.h is only available on GLIBC/UCLIBC

Include any other standard C library file. This will indirectly
include features.h and then one can check for GNU libc. This is
fixing the build for FreeBSD.
freebsd: The context is typedefed to mcontext_t on i386

Fix the compiling by using the right type. At least on FreeBSD
11-CURRENT it mcontext_t.
Merge pull request #56 from zecke/pharo-freebsd

Fix compilation on FreeBSD