Windows: Permit build system to update symbol store

Windows: Permit build system to update symbol store

Microsoft Debugging Tools for Windows, Visual Studio Debugger,
and SysInternals tools can all make use of a Symbol Server.
  http://msdn.microsoft.com/en-us/magazine/cc163563.aspx
The commit adds functionality to the build system to automatically
add binaries and symbols to a symbol store during the build.
This functionality is only enabled if two environment variables
are defined:

  SYMSTORE_EXE  - specifies the location of symstore.exe
  SYMSTORE_ROOT - specifies the location of the symbol store

an optional environment variable permits an arbitrary comment
to be added to the symbol store history file.

  SYMSTORE_COMMENT - arbitrary text to be added to the history

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/324
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>