CMake: include directories not propagated with target

[KerstinKeller]

When you build and install libde265 with CMake, you cannot properly use the library after using target_link_libraries, because the include directories are not propagated properly.

The reason is this line:

libde265/libde265/CMakeLists.txt

Line 132 in 0b1752a

install(FILES ${libde265_public_headers} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/libde265)

You also need to state where the header files can be found in the installed case, for an example like this:
https://github.com/eclipse-ecal/tcp_pubsub/blob/4ceafb6b55ce41149ed9473dae7e8ea37235a3cc/tcp_pubsub/CMakeLists.txt#L96-L103

Sample CMakeLists.txt file to use libde265:

cmake_minimum_required(VERSION 3.15)
project(libde265PackageTest CXX)

find_package(libde265 REQUIRED)
add_executable(libde265_test libde265_test.cpp)
target_link_libraries(libde265_test PRIVATE  de265)

Building a sample then fails:

fatal error: de265-version.h: No such file or directory

Would you accept a PR to address this issue?
At the same time I would recommend to export targets into a namespace so they can be linked with libde265::libde265, to make clear to CMake that this target is imported and then it gets better diagnostics if it cannot be found.

[farindk]

Thank you.
Sure, if you have a PR ready, I would integrate this. I don't know all CMake details, so every input is welcome.
Please note that we currently can only use CMake up to 3.10 (preferably a bit lower) because of the packaging for old distributions.