Make Haiku lie and pretend it's unix.

src/build/Common.gmake

@@ -288,6 +288,9 @@ endif
 ifeq ($(os),netbsd)	# NetBSD
  CFLAGS	+=	-D__unix__ -I/usr/pkg/include
 endif
+ifeq ($(os),haiku)
+ CFLAGS	+=	-D__unix__
+endif
 ifeq ($(os),darwin)
  CFLAGS +=  -D__unix__ -fno-common -D__DARWIN__
 endif