Improvements to coroutining support. These changes allow us to do

Estimated hours taken: 20

Improvements to coroutining support. These changes allow us to do
provide io primatives that cause the Mercury context to suspend rather
than causing the engine to block.

configure.in:
	Test to see if we can handle contexts that block on IO
	using select().

compiler/pragma_c_gen.m:
	Include the predicate name in the calls to MR_OBTAIN_GLOBAL_C_LOCK
	and MR_RELEASE_GLOBAL_C_LOCK for improved debugging.

	Fix a bug where the global lock was not being released when
	semidet pragma c code failed.

runtime/mercury_thread.h:
	Change the global lock macros to include the message generated
	by the changes to pragma_c_gen.

library/char.m:
library/std_util.m:
	include `thread_safe' in the flags for a couple of pragma c
	definitions that seem to have missed out.

runtime/mercury_context.{c,h}:
	Add a list of "pending" contexts that are blocked on a
	file descriptor. When the runqueue becomes empty, we call
	select on all the pending contexts.

	Move schedule from the header file to the c file (changing
	it from a macro to a function) for easier debugging at a
	slight performance cost.

	TODO: add a nonblocking call to select so that we can poll
	for io from time to time rather than waiting till there is
	nothing else to do.

runtime/mercury_reg_workarounds.{c,h}:
	Make functions that forward to the FD_* macros, which on Linux
	attempt to use registers that we've already grabbed. Aarrggh!

runtime/mercury_thread.c:
	Tidy up some of the conditional compilation.

runtime/mercury_types.h:
	Remove the definition of SpinLock since we're not using them
	and are not likely to any time soon.

runtime/mercury_thread.h

@@ -15,7 +15,7 @@
   #if defined(MR_DIGITAL_UNIX_PTHREADS)
     #define MR_MUTEX_ATTR		pthread_mutexattr_default
     #define MR_COND_ATTR		pthread_condattr_default
-    #define MR_THREAD_ATTR	pthread_attr_default
+    #define MR_THREAD_ATTR		pthread_attr_default
   #else
     #define MR_MUTEX_ATTR		NULL
     #define MR_COND_ATTR		NULL
@@ -27,7 +27,7 @@
   typedef pthread_mutex_t	MercuryLock;
   typedef pthread_cond_t	MercuryCond;
 
-  #if 0
+  #ifndef MR_DEBUG_THREADS
 	/*
 	** The following macros should be used once the
 	** use of locking in the generated code is considered
@@ -57,11 +57,11 @@
 	** predicates which are not thread-safe.
 	** See the comments below.
 	*/
-  #define MR_OBTAIN_GLOBAL_C_LOCK()	MR_mutex_lock(&MR_global_lock, \
-						"pragma c code");
+  #define MR_OBTAIN_GLOBAL_C_LOCK(where)	MR_LOCK(&MR_global_lock, \
+							(where));
 
-  #define MR_RELEASE_GLOBAL_C_LOCK()	MR_mutex_unlock(&MR_global_lock, \
-						"pragma c code");
+  #define MR_RELEASE_GLOBAL_C_LOCK(where)	MR_UNLOCK(&MR_global_lock, \
+							(where));
 
   #if defined(MR_DIGITAL_UNIX_PTHREADS)
     #define MR_GETSPECIFIC(key) 	({		\