FindGTK2: handle older libsigc++ versions
Older versions of libsigc++ do not have this header. It can be assumed that they are too old for C++11 support (which is what we're checking for).
Fixes #16654 (closed).
Older versions of libsigc++ do not have this header. It can be assumed that they are too old for C++11 support (which is what we're checking for).
Fixes #16654 (closed).