platformio.ini:
[env:my_env]
platform = ...
extra_scripts = extra_script.py
extra_script.py (place it near platformio.ini):
Import("env")
# General options that are passed to the C and C++ compilers
env.Append(CCFLAGS=["flag1", "flag2"])
# General options that are passed to the C compiler (C only; not C++).
env.Append(CFLAGS=["flag1", "flag2"])
# General options that are passed to the C++ compiler
env.Append(CXXFLAGS=["flag1", "flag2"])