kwebkitpart should be built as a MODULE instead of SHARED in order that the library prefix is omitted so as to match the X-KDE-Library path in kwebkitpart.desktop. Patch: https://github.com/cygwinports/kwebkitpart/blob/51ed5132903054b02ba81b927d73cb8e1a84dd1b/1.3-cmake-module.patch P.S. an actual release for KF5 would be really nice.
Updated patch: https://github.com/cygwinports/kwebkitpart/blob/9230a67b26b673808faf837d50a71bfd5423c50e/1.3-cmake-module.patch
Done with https://cgit.kde.org/kwebkitpart.git/commit/?id=03f4ff965e35e37d8d8006ba0f70d0fdfedbb01d