Fix map loading on OS X.

@DanielGibson pointed out that _SC_PAGESIZE itself is too big,
_SC_PAGESIZE - 1 is correct. Also apply a small performance
optimization by querying _SC_PAGESIZE only once.

src/backends/unix/shared/hunk.c

@@ -99,7 +99,8 @@ Hunk_End(void)
 	n = (byte *)mremap(membase, maxhunksize, curhunksize + sizeof(size_t), 0);
 #else
  #ifndef round_page
- #define round_page(x) ((((size_t)(x)) + sysconf(_SC_PAGESIZE)) & ~(_SC_PAGESIZE))
+ size_t page_size = sysconf(_SC_PAGESIZE);
+ #define round_page(x) ((((size_t)(x)) + page_size-1) & ~(page_size-1))
  #endif
 
 	size_t old_size = round_page(maxhunksize);