fix compilation with gc 7.1

* configure.ac: Add checks for GC_gcollect_and_unmap and
  GC_get_unmapped_bytes.
* libguile/gc-malloc.c (scm_realloc): GC_gcollect() if we don't have
  GC_gcollect_and_unmap.
* libguile/gc.c (GC_get_heap_usage_safe): Likewise, don't
  GC_get_unmapped_bytes if the function doesn't exist.

configure.ac

@@ -1259,7 +1259,7 @@ save_LIBS="$LIBS"
 LIBS="$BDW_GC_LIBS $LIBS"
 CFLAGS="$BDW_GC_CFLAGS $CFLAGS"
 
-AC_CHECK_FUNCS([GC_do_blocking GC_call_with_gc_active GC_pthread_exit GC_pthread_cancel GC_allow_register_threads GC_pthread_sigmask GC_set_start_callback GC_get_heap_usage_safe GC_get_free_space_divisor])
+AC_CHECK_FUNCS([GC_do_blocking GC_call_with_gc_active GC_pthread_exit GC_pthread_cancel GC_allow_register_threads GC_pthread_sigmask GC_set_start_callback GC_get_heap_usage_safe GC_get_free_space_divisor GC_gcollect_and_unmap GC_get_unmapped_bytes])
 
 # Though the `GC_do_blocking ()' symbol is present in GC 7.1, it is not
 # declared, and has a different type (returning void instead of

libguile/gc-malloc.c

@@ -96,7 +96,11 @@ scm_realloc (void *mem, size_t size)
     return ptr;
 
   /* Time is hard: trigger a full, ``stop-the-world'' GC, and try again.  */
+#ifdef HAVE_GC_GCOLLECT_AND_UNMAP
   GC_gcollect_and_unmap ();
+#else
+  GC_gcollect ();
+#endif
 
   SCM_SYSCALL (ptr = realloc (mem, size));
   if (ptr)

libguile/gc.c

@@ -205,7 +205,11 @@ GC_get_heap_usage_safe (GC_word *pheap_size, GC_word *pfree_bytes,
 {
   *pheap_size = GC_get_heap_size ();
   *pfree_bytes = GC_get_free_bytes ();
+#ifdef HAVE_GC_GET_UNMAPPED_BYTES
   *punmapped_bytes = GC_get_unmapped_bytes ();
+#else
+  *punmapped_bytes = 0;
+#endif
   *pbytes_since_gc = GC_get_bytes_since_gc ();
   *ptotal_bytes = GC_get_total_bytes ();
 }