Prefer GenerateExportHeader to EXPORT_ALL_SYMBOLS

[mathstuf]

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In my experience, having finer control over exported symbols is much better than exporting all of them (either via the default behavior of GCC-and-friends or via WINDOWS_EXPORT_ALL_SYMBOLS). Among these benefits:

• smaller binary size (internal template instantiation names can be hidden)
• better inlining; if the compiler/LTO knows a symbol isn't exposed at all, it can be more aggressively inlined)
• less chance of someone using a symbol you didn't intend to expose (more a problem in C than C++)

Instead, GenerateExportHeader should be used to generate a header with _EXPORT macros to decorate classes, functions, and variables as being available to other libraries.

[drdanz]

:+1: Unfortunately we are dealing with lazy researchers who ignore this kind of issues, therefore EXPORT_ALL_SYMBOLS is the quick and dirty way to achieve the same result. :stuck_out_tongue_closed_eyes: