lib: remove named variadic macros in exported headers

lib: remove named variadic macros in exported headers

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

Since there is no way to force named variadic macros as extensions, use a
a standard __VA_ARGS__ with an extra dummy argument to format strings.

This commit prevents the following errors:

 error: ISO C does not permit named variadic macros

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