[regression] torsocks uses linux-specific tsocks_libc_accept4 on FreeBSD

See src/lib/torsocks.c.

Version 2.3.0

torsocks.c:237:2: error: use of undeclared identifier 'tsocks_libc_accept4'; did you mean 'tsocks_libc_accept'?
        tsocks_libc_accept4 = dlsym(libc_ptr, LIBC_ACCEPT4_NAME_STR);
        ^~~~~~~~~~~~~~~~~~~
        tsocks_libc_accept
./torsocks.h:411:8: note: 'tsocks_libc_accept' declared here
extern TSOCKS_LIBC_DECL(accept, LIBC_ACCEPT_RET_TYPE, LIBC_ACCEPT_SIG)
       ^
./torsocks.h:34:9: note: expanded from macro 'TSOCKS_LIBC_DECL'
        type (*tsocks_libc_##name)(sig);
               ^
<scratch space>:55:1: note: expanded from here
tsocks_libc_accept
^
torsocks.c:237:40: error: use of undeclared identifier 'LIBC_ACCEPT4_NAME_STR'
        tsocks_libc_accept4 = dlsym(libc_ptr, LIBC_ACCEPT4_NAME_STR);
                                              ^
torsocks.c:239:53: error: use of undeclared identifier 'tsocks_libc_accept4'; did you mean 'tsocks_libc_accept'?
                        !tsocks_libc_syscall || !tsocks_libc_execve || ! tsocks_libc_accept4) {
                                                                         ^~~~~~~~~~~~~~~~~~~
                                                                         tsocks_libc_accept