Using -fconstant-string-class=

[Nicola Pero]

GCC 3.0 now provides -fconstant-string-class to change the name of the
class used for constant strings.  I hope that will work, but I'm not sure
as I have not yet tested it. 

Anyway, my plan would be:

 1. test -fconstant-string-class in isolation to make sure it works;

 2. assuming it works, add support in gnustep-base for it.

2. would sketchy work something like as follows:

In core/base/configure.in we need to detect that the compiler supports  
-fconstant-string-class.
 
If that is the case:
 
* we add -fconstant-string-class to the flags in base.make, so they are
then used to compile everything containing the base library, and we
redirect all constant strings to NSConstantString.

* we do not compile NXConstantString in gnustep-base, but rather we
compile in the same code but with class name NSConstantString.

* we then need to update the code referring to NXConstantString to refer
to NSConstantString.  And that gives me an idea.

* In case client code needs to refer to the constant-string class, we
need to have a way to communicate to the client code which one is the
constant string class (because depending on the case, that would then
be NXConstantString or NSConstantString).  I suppose we could add a
`+constantStringClass' to NSString (or similar, better name chosen by
an Englishman), which returns the class.  Then, even inside the
gnustep-base library, we update all [NXConstantString class] calls to
be [NSString constantStringClass].

Any comments ?