Initial support for .par grades with MinGW64.

The pthreads-win32 has worked with 64-bit compilers since version 2.9.
This diff adds the initial support for the .par grades with MinGW64 and
pthreads-win32.

configure.ac:
	Set C compiler and linker flags for x86_64-w64-mingw32.
	(This is provisional; on my system the library appears
	to have a different name, but I'm not sure how normal
	this is.)

runtime/mercury_thread.h:
	Adjust the definition of the MR_SELF_THREAD_ID macro
	so that the integer it expands to is at least as big
	as a pointer.  (Needed for pthreads-win32 since thread
	ids are pointer values, not integers with that.)

runtime/mercury_thread.c:
	Avoid warnings in some debugging code.