Dangerous check for __func__

[jacereda]

Hi,

This will potentially break code including the header:

://github.com/warmcat/libwebsockets/blob/8674bf1585c0196f071eb6f0ae2184c9ca053301/include/libwebsockets.h#L146

__func__ is not a preprocessor macro, so this test will redefine the builtin mechanism potentially breaking macros that use func in the ways c99 allows.

It should probably check if we're on >=C99.