include_dir
Type: DirPath | Multiple: No | Default: “<Project>/include”
The path to project’s default header files. PlatformIO uses it for the
pio run command. The default value is include meaning an
include directory located under the root directory of the project. This
path will be added to CPPPATH of the build environment.
If you need to add extra include directories to CPPPATH scope, please use
build_flags with -I /path/to/extra/dir option.
This option can also be configured by the global environment variable
PLATFORMIO_INCLUDE_DIR.