How to add preprocessor build flags in PlatformIO

In order to define a preprocessor build flag in PlatformIO, add it to build_flags inĀ platformio.ini, prefixing it with -D:

build_flags = -DQUICKSPI_DEBUG_WRITES

or, in order to define it to a specific value, use

build_flags = -DQUICKSPI_DEBUG_WRITES=1