Automatically decide the value of --install-command (part 1).

Allow the possibility of the compiler automatically deciding what command(s) to
use to install files and directories instead of using the value of
--install-command (which defaults to "cp").  The eventual intention (NYI) is
that in the absence of a value for --install-command, the compiler will choose
an appropriate command based on the setting of --host-env-type (and possibly
some other stuff, for the example the OS version).

The rationale for all this to try to improve the current situation on Windows
(in the absence of Cygwin or MSYS) where file copying is a bit of a mess.  It
should also mean that the same Mercury installation can be more easily used
from both MSYS and cmd.exe.

NOTE: for the moment --install-command is still specified in Mercury.config.
This will be removed in a later change and the compiler will default to
choosing automatically.

compiler/globals.m:
	Add a type whose values describe how we should install files and
	directories.

	Add a field of that type, file_install_cmd, to the globals structure.

	Add access predicates.

compiler/handle_options.m:
	Set the new field in the globals.

compiler/file_util.m:
	Provide functions for constructing a command line for installing files
	and directories based on the value of the file_install_cmd field in
	the globals.

compiler/make.program_target.m:
compiler/module_cmds.m:
	Use the new functions in file_util.m.