CritBit-Tree implementation for pike

[Marcus Comstedt (ACROSS) (Hail Ilpalazzo!) @ Pike (-) developers forum]

Hi.  This code seems to depend on Linux-specific header files:

#### Making dynamic: post_modules/CritBit
Compiling post_modules/CritBit/tree_block_alloc.c
In file included from /tmp/pike/src/post_modules/CritBit/ptr_key.h:7:0,
                 from /tmp/pike/src/post_modules/CritBit/tree_block_alloc.c:9:
/tmp/pike/src/post_modules/CritBit/bitvector.h:4:20: fatal error: endian.h: No 
such file or directory
compilation terminated.

Please fix.  If you want to know the byteorder, just use the define
PIKE_BYTEORDER provided by the Pike include file global.h.