Move LIBS_SYSTEM from cpp to configure.

Move LIBS_SYSTEM from cpp to configure.

* configure.in (LIBS_SYSTEM): New output variable, replacing cpp.

* lib-src/Makefile.in (LIBS_SYSTEM) [!MSDOS]: Set with configure, not cpp.
(LIBS_SYSTEM) [MSDOS]: Use MSDOS_LIBS_SYSTEM.
(NOT_C_CODE): Remove, no longer used.
(config.h) [!MSDOS]: No longer include.
(LOADLIBES): Use LIBS_SYSTEM as a variable.

* src/Makefile.in (LIBS_SYSTEM): Set using configure, not cpp.
(LIBS_SYSTEM) [MSDOS]: Reset with MSDOS_LIBS_SYSTEM.
(LIBES): Use LIBS_SYSTEM as a variable.
* src/s/msdos.h (LIBS_SYSTEM): Rename to MSDOS_LIBS_SYSTEM.  Always define.
* src/s/aix4-2.h (LIBS_SYSTEM):
* src/s/freebsd.h (LIBS_SYSTEM):
* src/s/hpux10-20.h (LIBS_SYSTEM):
* src/s/sol2-6.h (LIBS_SYSTEM):
* src/s/unixware.h (LIBS_SYSTEM):
Move to configure.

* msdos/sed1v2.inp, msdos/sed3v2.inp (LIBS_SYSTEM): Edit to empty.