llvm mode shared segment fix for FreeBSD.
MAP_EXCL|MAP_FIXED is a (genuine) equivalent to Linux's MAP_FIXED_NOREPLACE.
#define CONST_PRIO 5
#ifndef MAP_FIXED_NOREPLACE
+# ifdef MAP_EXCL
+ #define MAP_FIXED_NOREPLACE MAP_EXCL|MAP_FIXED
+#else
#define MAP_FIXED_NOREPLACE MAP_FIXED
# endif
+#endif
#include <sys/mman.h>
#include <fcntl.h>
