This allows including C headers from C++. It sadly reduced the
diagnostics on C++ code as well, were there warning may make sense as
unintended side effect. We may be able to drop that later on, when more
C APIs are properly wrapped in native C++ APIs, so that C headers do no
longer need to be compatible with C++ compilers.
Added arduino-nano to BOARD_INSUFFICIENT_MEMORY/BOARD_BLACKLIST following suit
of how arduino-uno is marked, as arduino-nano is mostly an Uno in a different
form factor.
