* numbers.c (SCM_COMPLEX_VALUE): Use GUILE_I instead of _Complex_I

* numbers.c (SCM_COMPLEX_VALUE): Use GUILE_I instead of _Complex_I
directly, and only if GUILE_I was defined by the configure step.
(scm_log, scm_log10, scm_exp, scm_sqrt): Use SCM_COMPLEX_VALUE
code only if SCM_COMPLEX_VALUE is defined.

* configure.in (--without-64-calls): Use AC_MSG_CHECKING and
AC_MSG_RESULT instead of just echo.
(GUILE_I): New programs to try using _Complex_I or 1.0fi for the
imaginary unit.