Remove private-gc.h

Remove private-gc.h

* libguile/simpos.c (scm_getenv_int): Move here, from gc.c.

* libguile/private-gc.h: Remove, unused.

* libguile/simpos.h: Move scm_getenv_int declaration here.

* libguile/vm.c:
* libguile/gc.c: Adapt scm_getenv_int users.

* libguile/gc-malloc.c:
* libguile/load.c:
* libguile/script.c: Remove private-gc includes from non-users of
  scm_getenv_int.

* libguile/Makefile.am: Adapt.