Make sure __darwin__ is defined

Signed-off-by: David Goulet <dgoulet@ev0ke.net>
Define LIBC_SYSCALL_ for OS X

OS X has the same declaration as FreeBSD.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>
Use extern for tsocks_libc_* in torsocks.h

Define tsocks_libc_* in the relevant source files and define them as
external in torsocks.h to avoid duplicate symbol errors.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>