Use `SCM_DEPRECATED' in declarations of deprecated functions/variables.
* libguile/deprecated.c (SCM_BUILDING_DEPRECATED_CODE): New macro.
* libguile/async.c (SCM_BUILDING_DEPRECATED_CODE): Likewise.
* libguile/macros.c (SCM_BUILDING_DEPRECATED_CODE): Likewise.
* libguile/async.h, libguile/deprecated.h, libguile/eval.h,
libguile/gc.h, libguile/gc.h, libguile/macros.h, libguile/ports.h,
libguile/srfi-4.h, libguile/strings.h: Change declarations of
deprecated functions and variables to use `SCM_DEPRECATED' instead of
`SCM_API'.