various stuff if libssh2 was built with debug. If built without debug, the
function does nothing.
2 - configure --enable-debug is now enough to build a debug version (including
picky compiler options)
3 - internally, we no longer need/use #ifdef/#endif around all uses of the
_libssh2_debug() function/macro.
The scp.c example is the first application to test this new debug logging.
