[Cmake] Feature added

[Schumacher, Gordon]

I've added a feature, on my testbed, to allow builds on platforms with
severe limitations on command line lengths (er, namely, DJGPP DOS.)

In the style of GCC and many tools, I added a "pull parameters from a file"
command-line option, " at file".  I suspect that I will need this for the Unix
makefile style of builds under DJGPP, because the CMake calls that get
generated from the Makefile of the form -S$(CMAKE_CURRENT_SOURCE)
-O$(CMAKE_CURRENT_BINARY) -H$(CMAKE_SOURCE_DIR) -B$(CMAKE_BINARY_DIR)
get far too long once they are expanded into full directory names to be
allowable within a DOS command-line limit.  The other piece of the puzzle,
of course, will be for me to create a new DJGPP Makefile generator to
utilize this new switch.

Yes, I realize that I may be the only person to suffer from this
limitation... but hey, it wasn't tough to add the feature.

The reason that I bring it up, is that the "args" string vector in
cmake::SetArgs() was declared const, with good reason.  But the sensible
way for me to add new args onto it (from the file) is to blow away the
const-ness...

I can submit a patch of the changes I made in the morning.  For now I want
to go home :)