No O_DIRECTORY on osx

[mbucc]

Just use O_RDONLY? (That's what libressl did: libressl/openbsd#23)

c99    -c -o rsam.o rsam.c
rsam.c:31:5: warning: implicit declaration of function 'unlinkat' is invalid in C99 [-Wimplicit-function-declaration]
    unlinkat(home, ".sam.fifo", 0);
    ^
rsam.c:49:32: error: use of undeclared identifier 'O_DIRECTORY'
    home = open(pwent->pw_dir, O_DIRECTORY | O_RDONLY);

~/src/sam$ uname -rsp
Darwin 13.4.0 i386

[deadpixi]

Closing this in favor of #20, which we can use to track all of the building issues on Mac OS X.