wc: speedup counting of short lines

wc: speedup counting of short lines

Using a test file generated with:
  yes | head -n100M > 2x100M.txt

before> time wc -l 2x100M.txt
  real 0.842s
  user 0.810s
  sys  0.033s

after> time wc -l 2x100M.txt
  real 0.142s
  user 0.111s
  sys  0.031s

* src/wc.c (wc): Split the loop that deals with -l into 3.
The first is used at the start of the input to determine if
the average line length is < 15, and if so the second loop is
used to look for '\n' internally to wc.  For longer lines,
memchr is used as before to take advantage of system specific
optimizations which any outweigh function call overhead.
Note the first 2 loops could be combined, though in testing,
GCC 4.9.2 at least, wasn't sophisticated enough to separate
the loops based on the "check_len" invariant.
Note also __builtin_memchr() isn't significant here as
GCC currently only applies constant folding with that.
* NEWS: Mention the improvement.