Move LD_SWITCH_X_SITE_AUX from cpp to autoconf.

Move LD_SWITCH_X_SITE_AUX from cpp to autoconf.

* configure.in (LD_SWITCH_X_SITE_AUX): Use AC_SUBST only, not AC_DEFINE as well.
(LD_SWITCH_X_SITE_AUX_RPATH): New output variable.

* nt/config.nt (LD_SWITCH_X_SITE_AUX): Remove.

* src/s/gnu-linux.h (LD_SWITCH_SYSTEM): Use LD_SWITCH_X_SITE_AUX as a shell
variable.
* src/s/netbsd.h (LD_SWITCH_SYSTEM_tmp): Remove.
(LD_SWITCH_SYSTEM): Use $LD_SWITCH_X_SITE_AUX_RPATH.
* src/s/openbsd.h (LD_SWITCH_SYSTEM_tmp): Remove.
(LD_SWITCH_SYSTEM): Use $LD_SWITCH_X_SITE_AUX_RPATH instead of
LD_SWITCH_SYSTEM_tmp.
* src/Makefile.in (LD_SWITCH_X_SITE_AUX, LD_SWITCH_X_SITE_AUX_RPATH):
New variables, set by configure.