Makefile: Add libc for FreeBSD

When compiling for arm on FreeBSD __udivsi3 and __umodsi3 are not
found since they are in libc and not in libgcc. Adding -lc makes
it compile correctly.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
net.h: don't use the reserved name __unused

The __ are reserved. On FreeBSD __unused evaluated to
the attribure unused, causing a compilation failure.
Just use unused instead.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
compiler_gcc: do not redefine __gnu_attributes

GNU gcc allows extensions to be non compiler specific
by define __* for the attributes supported by gcc. Since
FreeBSD implements these, redefining them leads to warnings.
Therefore don't define these macro's when already defined.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>