feat: create cmake alias for consistent downstream usage (#522)

At the moment, downstream projects will have to differentiate based off the way the obtained sdbus-c++ in CMake. If they use `find_package`, the target they have to use is `SDBusCpp::sdbus-c++`, whereas the target is merely `sdbus-c++` if they use `FetchContent` (i.e. use without installation).

By creating an alias target, it is now always possible to use the `DBusCpp::sdbus-c++` target regardless of how the library was obtained.

Author: Krzmbrzl

CMakeLists.txt

@@ -216,6 +216,9 @@ target_link_libraries(sdbus-c++-objlib
             Threads::Threads)
 
 add_library(sdbus-c++)
+# Create alias to allow consistent target use regardless whether used in-source or found via find_package
+add_library(SDBusCpp::sdbus-c++ ALIAS sdbus-c++)
+
 target_include_directories(sdbus-c++ PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
                                             $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 set_target_properties(sdbus-c++