Add more configure stuff for LXT and FST.

 * Add AC_FUNC_FSEEKO.
 * Add --enable-pthread to write FST in parallel.
 * Add --enable-judy to use Judy instead of builtin Jenkins for FST.
Do AC_CHECK_LIB([Judy], ...) once.
Make new options clear that they applies to FST only.
Make new options disabled by default to minimize number of deps.
Use AX_PTHREAD.
Merge remote-tracking branch 'hiyuh/conf'
Fix following error on cygwin.

make.c:196:10: error: array subscript has type `char' [-Werror=char-subscripts]
          *p = tolower(*p);
          ^