By default, libjpeg-turbo 1.3.x and later have enabled the in-memory
source/destination manager functions from libjpeg v8 when emulating the
libjpeg v6b or v7 API/ABI, which has allowed operating system
distributors to provide those functions without adopting the
backward-incompatible libjpeg v8 API/ABI.
Prior to libjpeg-turbo 1.5.x, it made sense to allow users to disable
the in-memory source/destination manager functions at build time and
thus retain both backward and forward API/ABI compatibility relative to
libjpeg v6b or v7. Since then, however, we have introduced several new
libjpeg API functions that break forward API/ABI compatibility, so it no
longer makes sense to allow the in-memory source/destination managers to
be disabled. libjpeg-turbo only claims to be
backward-API/ABI-compatible, i.e. to allow applications built against
libjpeg or an older version of libjpeg-turbo to work properly with the
current version of libjpeg-turbo.
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