Add correct defines for threaded boehm-gc on linux.

gc_boehm.c

@@ -12,6 +12,23 @@
 #include <signal.h>
 #include "gc_ops.h"
 #define I_HIDE_POINTERS
+
+/*
+ * Citing boehm-gc's README.linux:
+ *
+ * 3a) Every file that makes thread calls should define GC_LINUX_THREADS and
+ *  _REENTRANT and then include gc.h.  Gc.h redefines some of the
+ *  pthread primitives as macros which also provide the collector with
+ *  information it requires.
+ */
+#ifdef __linux__
+# define GC_LINUX_THREADS
+
+# ifndef _REENTRANT
+#  define _REENTRANT
+# endif
+
+#endif
 #include <gc/gc.h>
 #include <gc/gc_typed.h>