Make Mercury compile with Visual Studio 2013 (MSVC 12.0).

Fix some problems that were preventing the system from compiling properly with
MSVC.

(1) Visual Studio 2013 no longer includes the files NtWin32.Mak and Win32.Mak
which are required for building the Boehm GC with MSVC.  (This appears to be an
issue with upstream Boehm as well.)  The workaround here is to reference the
copies of this files included with the Windows 7 SDK.  (This appears to be the
official Microsoft workaround for dealing with this problem for now.)

(2) Disable support for `--c-debug' when using MSVC.  Using this option causes
MSVC to emit .pdb (Program Data Base), that contain the symbol information for
an object files.  This causes a variety of problems:

   (i) parallel builds are contending for the vc*.pdb file shared by all the
   object files in a directory causing compilation to abort.  (Compiling with
   the -FS option is another solution to this -- it causes the compiler to
   serialize access to the .pdb file -- but at the cost of slowing down
   compilation.)

   (ii) .pdb files are not cleaned up by realclean; compilation will abort
   if a pdb file generated by a different version of MSVC is encountered.

   (iii) we don't install the .pdb files alongside the libraries anyway.
   This can be a source of linker warnings.  (And shutting up the Microsoft
   linker seems to be very difficult indeed ...)

   (iv) compiling with -Zi (apparently) inhibits some C compiler optimizations.

Compiling with the older -Z7 (MSVC 7 style debugging info) option is another
alternative.  Using that option causes MSVC to include symbol information in
the object files, instead of in a separate file.

For now (i.e. the 14.01 release), disabling `--c-debug' for MSVC is the
simplest way of addressing the above issues.

README.MS-VisualC:
	Describe the problem with NtWin32.Mak and Visual Studio 2013 and
	provide two possible workarounds.

	Mention that `--c-debug' is not currently supported with MSVC and how
	to re-enable it.

	Simplify some of the instructions for setting up the build environment.

configure.ac:
scripts/mgnuc.in:
	Disable `--c-debug' with MSVC.

scripts/prepare_install_dir.in:
	Copy all makefile fragments for nmake into the boehm_gc directory for
	the library grade installation.  (This is in case the user copies
	NtWin32.Mak etc into the boehm_gc directory as per the second of
	the workarounds in README.MS-VisualC.)

slice/Mmakefile:
compiler/Mmakefile:
deep_profiler/Mmakefile:
mfilterjavac/Mmakefile:
profiler/Mmakefile:
slice/Mmakefile:
	Clean up any .pdb files.  Note that one is generated for each
	executable (regardless of whether -Zi is given to cl or not).
 	In addition, if -Zi is given to cl, there will be file named
	vc<version>.pdb generated (where <version> is the version of MSVC being
	used.)
Avoid compiler warnings from MSVC in generated code.

runtime/mercury_float.h:
	Avoid warnings from MSVC about the types of the formal and actual parameters
	of calls to MR_float_from_dword differing.  The formal parameters have type
	MR_Word, but the Mercury compiler doesn't emit casts for the arguments.
	(For GCC and clang, MR_float_from_dword is defined as a macro and the
	necessary casts occur in its body.)  The fix is to rename the function
	version of MR_float_from_dword and make MR_float_from_dword a forwarding macro
	that inserts the necessary casts.
Avoid C compiler warnings on OS X with Xcode 5.

In the mgnuc script, do not use the name of the C compiler to determine its
type since on OS X with Xcode 5 the executable "gcc" is actually clang.
Instead use the value of C_COMPILER_TYPE determined by the configure script to
set C compiler type, as elsewhere in the Mercury system.

Incorrectly identifying the C compiler type was resulting in lots of
compilation warnings because we were using the warning flags intended for GCC
with clang.  Further warnings were being generated because GCC-only
optimization flags would also be passed to clang.

scripts/mgnuc.in:
	As above.
Update README.MacOS.

Update README.MacOS to cover version 10.9  (Xcode 5).

README.MacOS:
	Add a contents section to the top of this file.

	Merge the sections covering Mac OS X 10.5 - 10.8 into one section that
	also covers 10.9.

	The source distribution will no longer be built in the none.gc grade.

	Add separate sections discussing Xcode 4 and Xcode 5 specific issues.

        Mention that Homebrew and Fink are also a source of GCC packages for
   	Mac OS X (in addition to MacPorts).

	Mac OS 10.3 was only supported on PowerPC machines, not as we seem
	to suggest here also Intel machines.
Conform to changes in libhwloc's API (Bug #289)

The 'cpuset' functions in libhwloc have been made more generic and renamed
to 'bitmap' functions.  This patch updates the Mercury runtime system
appropriately.

runtime/mercury_context.c:
    As above.

configure.ac:
    We now require at least version 1.1 of libhwloc (previously we required
    at least 1.0).