Split C/C++ build flags

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"])