Set WITH_SDL in parent scope so xpdevplus can pick it up.

14 jobs for master in 12 minutes and 35 seconds (queued for 19 seconds)
Status Name Job ID Coverage
  Build
passed jsdoor-freebsd #31944
FreeBSD

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passed jsdoor-linux #31945
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passed jsdoor-windows #31947
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passed sbbs-freebsd #31939
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passed sbbs-linux #31934
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passed sbbs-windows #31937
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passed sexpots-freebsd #31946
FreeBSD

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passed sexpots-linux #31935
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passed sexpots-windows #31938
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passed syncdraw-freebsd #31942
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passed syncdraw-linux #31943
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passed syncterm-freebsd #31940
FreeBSD

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passed syncterm-linux #31936
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passed syncterm-windows #31941
FreeBSD

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