Fix SDK destination installation path for Windows platform.

It appears CMake has automatically prepended "Urho3D" to built-in CMAKE_INSTALL_PREFIX variable, so we don't need to prefix the path one more time. Also in the commits:
- Fix documentation on CMake module search path for Windows platform.
- Unified SDK destination installation for Windows and non-Windows platforms.
- Replace global include directories to SDL and export header to locally scoped include directories.
- Enhance Urho3D.pc generation to also provide additional compiler flags for DEBUG/RELEASE/RELWITHDEBINFO as comments/references.
Moved SDL header include dir to 'Engine' scope as it is being referenced by a few Engine sub-directories and not just Graphics.
Merge remote-tracking branch 'upstream/master'