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.
cmake: Generate relocatable buildsystem

This way I can generate it on my laptop and move the source to a
remote system without having the same user account there. This will
help in creating proper source packages for deb and rpm based
distributions.
bash: Do not assume bash is installed in /bin/bash

There are systems that do not provide a bash in the standard
installation. Use "/usr/bin/env bash" to execute a bash. This
is needed for FreeBSD.
freebsd: Add a working FreeBSD 10.0-RELEASE/11-CURRENT config

The ThirdPartyLibrary name handling is the most nasty part as it
shows issues with the inheritance and assuming "linux" = "unix".
Merge pull request #60 from JeanBaptisteArnaud/master

Push alien fix
Merge pull request #56 from zecke/pharo-freebsd

Fix compilation on FreeBSD
Merge pull request #57 from zecke/cmake-relocatable

cmake: Generate relocatable buildsystem
Merge pull request #59 from zecke/freebsd-build

Freebsd build