BACKWARDS_COMPATIBILITY and BACKWARDS_COMPATIBLE

[Petr M. <mi...@ph...>]

I've just noticed many error messages from gnuplot running under Octave 
... that's because "set nopolar", "set notics" etc does no longer work 
even though the backwards compatibility is enabled by default...

The bug: there is a mix of both keywords BACKWARDS_COMPATIBILITY and 
BACKWARDS_COMPATIBLE in the source code but it should be the same.

Someone knows why/when/where this change happened?
I guess it should be enough to fix the constant name in configure.in.

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