Using the latest (as of today) gcc-arm-embedded toolchain caused two build
failures similar to:
error: unknown type name 'cookie_io_functions_t'
These custom IO functions are now protected by a define, so define _GNU_SOURCE
which enables all such areas. This is a libc issue.
Signed-off-by: Trevor Woerner <twoerner@gmail.com>
a5641ab63e