Ticket #2998: configure.ac cleanups.

Remove tests whose results are not used:
  * AC_CHECK_HEADERS of unistd.h, fcntl.h, sys/time.h, sys/types.h
  * AC_HEADER_MAJOR
  * AC_HEADER_TIME
  * AC_HEADER_DIRENT
  * AC_CHECK_FUNCS of isascii, statfs, sysconf, tcgetattr, tcsetattr, ftruncate
  * AC_FUNC_STRCOLL

AC_C_CONST is obsolete, today it is safe to assume all C compilers
support at least C89.

The INTERACTIVE UNIX that needed AC_ISC_POSIX is long dead.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>