C99, variadic macros, and rbtree.c

[Per Hedbor () @ Pike (-) developers forum]

C99 would be nice, there is a lot of nice additions.

And we could remove a lot of the various ATTRIBUTE() style macros.
And there are predefined types for integer-with-x-bits
(fastest-with-or-at-least x bits etc). That removes another bunch of
configure tests.

Also, there is of course a bunch of new alway-available functions.

And // is now valid. As is variable declarations in the code. And in
for(>here<;;).

And the rounding functionality of % has been standardised.
Unfortunately in the other way from the one pike uses. :)