* __scm.h (SCM_ALLOW_INTS_ONLY): Removed.

* __scm.h (SCM_ALLOW_INTS_ONLY): Removed.
(SCM_NONREC_CRITICAL_SECTION_START,
SCM_NONREC_CRITICAL_SECTION_END, SCM_REC_CRITICAL_SECTION_START,
SCM_REC_CRITICAL_SECTION_END): New macros.
(SCM_CRITICAL_SECTION_START/END): Defined here.

* eval.c: Insert SOURCE_SECTION_START / SOURCE_SECTION_END around
the three calls to scm_m_expand_body.

* gc.h: #include "libguile/pthread-threads.h";
(SCM_FREELIST_CREATE, SCM_FREELIST_LOC): New macros.

* gc.c (scm_i_freelist, scm_i_freelist2): Defined to be of type
scm_t_key;

* gc.c, gc-freelist.c, inline.h: Use SCM_FREELIST_LOC for freelist
access.

* gc-freelist.c (scm_gc_init_freelist): Create freelist keys.

* gc-freelist.c, threads.c (really_launch): Use
SCM_FREELIST_CREATE.

* gc-malloc.c (scm_realloc, scm_gc_register_collectable_memory):

* gc.c (scm_i_expensive_validation_check, scm_gc,
scm_gc_for_newcell): Put threads to sleep before doing GC-related
heap administration so that those pieces of code are executed
single-threaded.  We might consider rewriting these code sections
in terms of a "call_gc_code_singly_threaded" construct instead of
calling the pair of scm_i_thread_put_to_sleep () and
scm_i_thread_wake_up ().  Also, we would want to have as many of
these sections eleminated.

* init.c (scm_init_guile_1): Call scm_threads_prehistory.

* inline.h: #include "libguile/threads.h"

* pthread-threads.h: Macros now conform more closely to the
pthreads interface.  Some of them now take a second argument.

* threads.c, threads.h: Many changes.

* configure.in: Temporarily replaced "copt" threads option with new
option "pthreads".
(USE_PTHREAD_THREADS): Define if pthreads configured.