DEFINE_SYMBOL

From Get docs
Cmake/docs/3.21/prop tgt/define symbol


DEFINE_SYMBOL

Define a symbol when compiling this target's sources.

DEFINE_SYMBOL sets the name of the preprocessor symbol defined when compiling sources in a shared library. If not set here then it is set to target_EXPORTS by default (with some substitutions if the target is not a valid C identifier). This is useful for headers to know whether they are being included from inside their library or outside to properly setup dllexport/dllimport decorations.

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https://cmake.org/cmake/help/v3.21/prop_tgt/DEFINE_SYMBOL.html