Move LD_SWITCH_SYSTEM from cpp to configure.

Move LD_SWITCH_SYSTEM from cpp to configure.

* configure.in (LD_SWITCH_SYSTEM): Set with configure, not cpp.
Merges logic from src/s/* and src/Makefile.in.
(LD_SWITCH_SYSTEM_TEMACS): New output variable.

* src/Makefile.in (LD_SWITCH_SYSTEM): Set with configure, not cpp.
(LD_SWITCH_SYSTEM_EXTRA): New variable, set by configure.
(TEMACS_LDFLAGS): Use $LD_SWITCH_SYSTEM and $LD_SWITCH_SYSTEM_EXTRA,
move out of cpp section.
* src/s/freebsd.h (LD_SWITCH_SYSTEM):
* src/s/gnu-linux.h (LD_SWITCH_SYSTEM):
* src/s/netbsd.h (LD_SWITCH_SYSTEM):

* admin/CPP-DEFINES: Remove LD_SWITCH_SYSTEM.