XviD: set C version to 1999

[herbygillot]

In C23, bool is a built-in keyword. Guard the typedef int bool; in encoder.h so it is only used with older C standards.

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[reneeotten]

probably caused by the autoconf update - see @ryandesign message on the mailinglist. Instead of patching you could probably also set upstream's required, and probably older, standard than C23.

[herbygillot]

probably caused by the autoconf update - see @ryandesign message on the mailinglist. Instead of patching you could probably also set upstream's required, and probably older, standard than C23.

Good point, thanks @reneeotten.