If you compare KDE's libssh 0.5.3 src/agent.c ftp://winkde.org/kde/ports/win32/releases/stable/latest/libssh-x86-mingw4-0.5.3-src.tar.bz2
To the vanilla libssh 0.5.3 src/agent.c http://git.libssh.org/projects/libssh.git/tree/src/agent.c?id=libssh-0.5.3
You'll see that KDE patched it to add support for Pageant as an SSH agent for Windows.
There's also winpgntc.c and winpgntc.h included in the KDE version, but not in the vanilla version.
Our libssh 0.5.5 build doesn't have Pageant support: http://git.libssh.org/projects/libssh.git/tree/src/agent.c?id=libssh-0.5.5
And neither does libssh 0.6.3: http://git.libssh.org/projects/libssh.git/tree/src/agent.c?id=libssh-0.6.3 Or the current (2014-03-16) HEAD: http://git.libssh.org/projects/libssh.git/tree/src/agent.c
So right now I need to find the applied patch they applied.
The src.tar.bz2 file contains patches, but only for CVE vunlerabilities. I do not see .patch files for this feature, it is already applied to the source.