The following keywords can be used in a platform.ini as in the following example line:

build_flags = -DANABRID_DEBUG_INIT -DANABRID_UNSAFE_INTERNET -DANABRID_SKIP_DHCP

Here is a (not yet comprehensive) list of keywords:

ANABRID_UNSAFE_INTERNET: Skips to build any user/password authentification into the code
ANABRID_SKIP_DHCP: Skips to build the DHCP client at startup (use this if you want to enforce using static IPv4)
ANABRID_ENABLE_GLOBAL_PLUGIN_LOADER: Enables the global storage plugin loader, which reserves a few kB of precious global variable storage for the plugin feature. This option currently also enables the plugin system as-is.
ANABRID_ENABLE_OTA_FLASHING: Enables over-the-air flashing, i.e. firmware update over TCP/IP instead of traditional Teensy USB.
ANABRID_DEBUG_INIT: Print more debugging messages at startup
ANABRID_DEBUG_... also for various other modules

Warning: If you enable the plugin system or OTA flashing without enabling the authentification system, anybody with network access can run arbitrary code on your microcontroller. That is, the combination -DANABRID_UNSAFE_INTERNET -DANABRID_ENABLE_GLOBAL_PLUGIN_LOADER and/or -DANABRID_UNSAFE_INTERNET -DANABRID_ENABLE_OTA_FLASHING is dangerous.