On day 34, the following line was added to handmade_platform.h:

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    typedef size_t memory_index;


This causes a compilation error with g++:

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In file included from src/handmade.h:22:0,
                 from src/handmade.cc:9:
src/handmade_platform.h:27:9: error: 'size_t' does not name a type


Including stddef.h alongside the existing stdint.h include fixes that.