Use `SCM_DEPRECATED' in declarations of deprecated functions/variables.

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'.