The BuildDef.hpp file is an automatically generated file mostly used in generating production copies of your project. There are currently only 2 possible definitions available to define in the BuildDef.hpp file.

PRODUCTION

The PRODUCTION macro specifies if the project is compiled for production. In normal use, this macro should be explicitly undefined like this:

#undef PRODUCTION

but when generating a production build, the file briefly changes to define it

When this macro is defined, many developer related components such as the Editor will be excluded from compilation for 2 reasons

DEVELOPMENT

The DEVELOPMENT macro will activate the developer mode that should only be used by engine developers. Normally it isn't defined but if you define it yourself you will compile with the debug code allowing you to access: