Move C_SWITCH_MACHINE, C_SWITCH_SYSTEM from cpp to autoconf.

Move C_SWITCH_MACHINE, C_SWITCH_SYSTEM from cpp to autoconf.

* configure.in (C_SWITCH_MACHINE, C_SWITCH_SYSTEM): New output
variables, replacing c_switch_machine, c_switch_system.

* src/s/aix4-2.h (C_SWITCH_SYSTEM):
* src/m/alpha.h (C_SWITCH_MACHINE):
Move to configure.in.
* src/Makefile.in (C_SWITCH_MACHINE, C_SWITCH_SYSTEM):
New variables, set by configure.
(ALL_CFLAGS): Use $C_SWITCH_MACHINE and $C_SWITCH_SYSTEM in place of
$c_switch_machine and $c_switch_system.

* lib-src/Makefile.in: Use @C_SWITCH_SYSTEM@, @C_SWITCH_MACHINE@
rather than @c_switch_system@, @c_switch_machine@.