osx: erroneous polyfill for kFSEventStreamCreateFlagNoDefer for OS X < 10.7

[ci42]

Commit 0f5c28b introduced a kFSEventStreamCreateFlagNoDefer polyfill in src/unix/internal.h for OS X < 10.7. But kFSEventStreamCreateFlagNoDefer is already defined in

/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/FSEvents.h

since 10.5, which causes the following build failure on 10.6 (probably on 10.5 too):

/src/unix/internal.h:246:18: error: redefinition of 'kFSEventStreamCreateFlagNoDefer' as different kind of symbol
static const int kFSEventStreamCreateFlagNoDefer = 0x00000002;
                 ^
/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/FSEvents.h:237:3: note: previous definition is here
  kFSEventStreamCreateFlagNoDefer = 0x00000002,
  ^
1 error generated.