Move UNEXEC definition to autoconf.

Move UNEXEC definition to autoconf.
* configure.in (unxec): Do not define and substitute.
(UNEXEC_OBJ): New output variable, replaces cpp UNEXEC.

* msdos/sed1v2.inp (UNEXEC_OBJ): Use UNEXEC_OBJ instead of unexec.

* src/s/usg5-4.h (UNEXEC): Remove, move to configure.in.
* src/s/sol2-10.h (UNEXEC):
* src/s/irix6-5.h (UNEXEC):
* src/s/hpux10-20.h (UNEXEC):
* src/s/gnu-linux.h (UNEXEC):
* src/s/darwin.h (UNEXEC):
* src/s/cygwin.h (UNEXEC):
* src/s/bsd-common.h (UNEXEC):
* src/s/aix4-2.h (UNEXEC):
* src/m/alpha.h (UNEXEC): Likewise.
* src/Makefile.in (UNEXEC_OBJ): Define using @UNEXEC_OBJ@.