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 on Windows. On POSIX platforms, this can optionally be used to control the visibility of symbols. CMake provides support for such decorations with the :module:`GenerateExportHeader` module.