build: resolve a -Wunused-function compiler warning

[jengelh]

posixSocket.cpp:67:7: warning: ‘char*
{anonymous}::vmime_strerror_r_result(int, char*)’ defined but not used [-Wunused-function]
   67 | char* vmime_strerror_r_result(int /* res */, char* buf) {

It is to be expected that one of the two r_result functions is going unused, depending on whichever platform we are currently building on.

[vincent-richard]

Hello! If I'm not mistaken, attribute is specific to GCC, and the standard way of doing it with [[maybe_unused]] is available in C++17 only. Maybe we could do some check at configure and use a #define depending on what we detect?

[jengelh]

The [[ ]] attribute syntax is C++11, so we can use it in principle, but compiler may nag about unknown attributes so I slapped it into a __GNUC__-based define to get this over with.