[MPlayer-dev-eng] Fix configure after r25139

[Piotr Kaczuba]

Revision r25139 has broken configure at two places.

At the first place, you need to remove the quotes because the configure 
function "not" is no longer and thus no eval is applied to the argument 
any more. One can put the variable $_libavdecoders in quotes, though.

At the second place, the "if" clause is a must because later on a 
matching "else" follows.

Piotr Kaczuba
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