fixes for the detection of GC_start_call_back

- HAVE_GC_SET_START_CALLBACK was defined unconditionally
- there was no AC_DEFINE if system_boehm was false
- there was a typo in one usage site of this macro

see 6b754564f1

src/aclocal.m4

@@ -948,6 +948,8 @@ if test "${enable_boehm}" = auto -o "${enable_boehm}" = system; then
  if test "${system_boehm}" = "yes"; then
    AC_CHECK_LIB( [gc], [GC_set_start_callback],
                  [AC_DEFINE(HAVE_GC_SET_START_CALLBACK)], [] )
+ else
+  AC_DEFINE(HAVE_GC_SET_START_CALLBACK)
  fi
  AC_MSG_CHECKING( [whether we can use the existing Boehm-Weiser library] )
  AC_MSG_RESULT( [${system_boehm}] )

src/c/alloc_2.d

@@ -32,7 +32,7 @@
 #ifdef GBC_BOEHM
 
 static void (*GC_old_start_callback)(void) = NULL;
-#ifdef HAVE_GC_START_CALLBACK
+#ifdef HAVE_GC_SET_START_CALLBACK
 extern void GC_set_start_callback(void *);
 extern void *GC_get_start_callback(void *);
 #else

src/configure

@@ -5970,6 +5970,9 @@ if test "x$ac_cv_lib_gc_GC_set_start_callback" = xyes; then :
 
 fi
 
+ else
+  $as_echo "#define HAVE_GC_SET_START_CALLBACK 1" >>confdefs.h
+
  fi
  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether we can use the existing Boehm-Weiser library " >&5
 $as_echo_n "checking whether we can use the existing Boehm-Weiser library ... " >&6; }

src/h/config.h.in

@@ -309,7 +309,7 @@ typedef unsigned char ecl_base_char;
 #endif
 
 /* GC_set_start_callback						*/
-#define HAVE_GC_SET_START_CALLBACK
+#undef HAVE_GC_SET_START_CALLBACK
 
 /*
  * SYSTEM FEATURES: