check for iconveh values at configure-time

check for iconveh values at configure-time

* configure.ac: Check for the iconveh values here, instead of relying on
  gen-scmconfig to know them.  That doesn't work in general because
  gen-scmconfig runs on the build machine, not the target machine.

* libguile/Makefile.am (gen-scmconfig.$(OBJEXT)): Revert rule to the
  revision before 533d8212.

* libguile/gen-scmconfig.h.in (SCM_I_GSC_ICONVEH_ERROR):
  (SCM_I_GSC_ICONVEH_QUESTION_MARK):
  (SCM_I_GSC_ICONVEH_ESCAPE_SEQUENCE):
* libguile/gen-scmconfig.c: Use configure-time substitutions to set
  SCM_ICONVEH_ERROR_HANDLER et al.