Merge pull request #1 from tmetsch/master

Buffered output hanlding appears to be broken on macOS. The -B flag is
disabled in dtrace.c ()
and compiling dtrace.c with that flag enabled results in the same crash
that I see with python-dtrace:
```
* thread tmetsch#1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x7fb100000000)
  * frame #0: 0x00007fff6f83de52 libsystem_platform.dylib`_platform_strlen + 18
    frame tmetsch#1: 0x00007fff6f6d9891 libsystem_c.dylib`__vfprintf + 5379
    frame tmetsch#2: 0x00007fff6f6ffad3 libsystem_c.dylib`__v2printf + 475
    frame tmetsch#3: 0x00007fff6f6e5ee7 libsystem_c.dylib`_vsnprintf + 417
    frame tmetsch#4: 0x00007fff6f6e5f90 libsystem_c.dylib`vsnprintf + 68
    frame tmetsch#5: 0x00007fff6d3650e3 libdtrace.dylib`dt_printf + 524
    frame tmetsch#6: 0x00007fff6d33da7f libdtrace.dylib`dt_consume_cpu + 2536
    frame tmetsch#7: 0x00007fff6d33c9e0 libdtrace.dylib`dtrace_consume + 1090
    frame tmetsch#8: 0x00007fff6d366534 libdtrace.dylib`dtrace_work + 116
    frame tmetsch#9: 0x000000010d6be9ff dtrace2`main(argc=4, argv=0x00007ffee2547250) at dtrace2.c:1834:17
    frame tmetsch#10: 0x00007fff6f647cc9 libdyld.dylib`start + 1
```

To work around this crash, output to stderr for now.