Fix detection of pthreads-win32.

The check for the pthreads-win32 implementation did not work because it
compiled a test program that was designed to be run, but did not run it.
Running a test program is undesirable for cross-compilation anyway.

m4/mercury.m4:
	Replace MERCURY_HAVE_PTHREADS_WIN32 test with one based only on
	successful compilation.
Remove unnecessary detection of sync term size during configure.

The size of a sync term must be kept synchronised between the runtime
and the compiler.  The size is "detected" during configure by running
a small program whose output would always be the same.  It doesn't work
for cross-compilation so a value is hard-coded in configure_mingw_cross,
which is liable to be missed if the sync term ever changes.

configure.ac:
	Hard code sync term size as it measures a number of detstack
	slots, which does not depend on architecture.

runtime/mercury_conf.h.in:
	Define MR_SYNC_TERM_SIZE.

runtime/mercury_std.h:
	Add MR_STATIC_ASSERT macro.

runtime/mercury_context.h:
	Check at compile time that the value of MR_SYNC_TERM_SIZE
	from configure.ac matches the real size.

tools/configure_mingw_cross:
	Delete mercury_cv_sync_term_size as no longer needed.
Support 64-bit Windows target in configure_mingw_cross.

Add support for MinGW-w64 cross-compilers targeting 64-bit Windows
in configure_mingw_cross.

tools/configure_mingw_cross:
	If the host triplet begins with "x86_64" then assume
	configuration values for a 64-bit target.

README.MinGW-cross:
	Update documentation.

NEWS:
	Announce change.
Match extended host triplets from MXE project.

The MXE project extends target triplets with dot-separated suffixes on
their unstable branch, e.g. "x86_64-w64-mingw32.static".

configure.ac:
compiler/Mmakefile:
	Relax two pattern matches for the extra suffixes.

	Delete ineffective assignment.
Add warnings about issues with 64-bit Windows and low-level C grades.

README.MinGW:
README.MinGW-cross:
	As above.