Move SYSTEM_TYPE from src/s to configure

Move SYSTEM_TYPE from src/s to configure

* configure.in (SYSTEM_TYPE): New AC_DEFINE.

* msdos/sed2v2.inp (SYSTEM_TYPE): Set it.

* nt/config.nt (SYSTEM_TYPE): Define it.

* src/s/aix4-2.h, src/s/bsd-common.h, src/s/cygwin.h, src/s/darwin.h:
* src/s/gnu-kfreebsd.h, src/s/gnu-linux.h, src/s/gnu.h, src/s/hpux10-20.h:
* src/s/irix6-5.h, src/s/ms-w32.h, src/s/msdos.h, src/s/template.h:
* src/s/usg5-4-common.h: Remove SYSTEM_TYPE.