fix code that causes warnings on gcc 4.6

fix code that causes warnings on gcc 4.6

* libguile/arrays.c (scm_i_read_array):
* libguile/backtrace.c (display_backtrace_body):
* libguile/filesys.c (scm_readdir)
* libguile/i18n.c (chr_to_case):
* libguile/ports.c (register_finalizer_for_port):
* libguile/posix.c (scm_nice):
* libguile/stacks.c (scm_make_stack): Clean up a number of
  set-but-unused vars.  Thanks to Douglas Mencken for the report.

* libguile/numbers.c (scm_log, scm_exp): Fix a few #if cases that should
  be #ifdef.