lib: work around unnamed structs/unions

lib: work around unnamed structs/unions

Exported header files used by applications should allow the strictest
compiler flags. Language extensions used in many places must be explicitly
marked to avoid warnings and compilation failures.

Unnamed structs/unions are allowed since C11, however many compiler
versions do not use this mode by default.

This commit prevents the following errors:

 error: ISO C99 doesn't support unnamed structs/unions
 error: struct has no named members

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>