Add DISABLE_MKSTEMP_DEFINE support

This macro as written won't compile on mingw32 due to the attributes
on _mktemp().  This could be worked around, but it's easier to just
disable it since I don't need it, and nobody else is building with
mingw32.
14 jobs for dailybuild_linux-x64 in 9 minutes and 28 seconds (queued for 14 seconds)
Status Name Job ID Coverage
  Build
passed jsdoor-freebsd #31286
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passed jsdoor-linux #31287
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passed jsdoor-windows #31289
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passed sbbs-freebsd #31281
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passed sbbs-linux #31276
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passed sbbs-windows #31279
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passed sexpots-freebsd #31288
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passed sexpots-linux #31277
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passed sexpots-windows #31280
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passed syncdraw-freebsd #31284
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passed syncdraw-linux #31285
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passed syncterm-freebsd #31282
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passed syncterm-linux #31278
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passed syncterm-windows #31283
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