Preprocessor Macros and Compilation Control

[michaelkamprath]

This is a major feature upgrade that enables preprocessor macro definitions using the #define <symbol> <value> syntax and compilation control using preprocessor #if / #elif / #else / #endif directives. See documentation for more information.

Also added several minor features documented in the CHANGELOG.

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This discussion was created from the release Preprocessor Macros and Compilation Control.