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.)