Equalize include flag and warning flags with platform.txt

[maxgerhardt]

PR #43 changed some -I includes to -isystem includes so that we don't get warnings from the SDK and ArduinoCore-API code that we don't touch.

Further, the Arduino core builds with warning flags (-Wall -Wextra) that PlatformIO doesn't use.

All these flags should be equalized.

A possible caveat with changing -I to -isystem flags in the PlatformIO system is however that when moving folders from the CPPPATH to CCFLAGS (-isystem flags), they might disappear from the include search directory in generated IntelliSense configuration files (c_cpp_properties.json). This would greatly limit IDE usefullness when using PlatformIO. If that is the case, an issue in the PlatformIO core must be opened (or accepted that we can't change -I to -isystem comfortably yet).