Add export support

This makes sdbus-c++ consumable from the build tree without the need for an explicit installation. This can be very handy for e.g. testing purposes. Crucially, downstream projects will depend on upstream to properly export everything. Otherwise, they can't be built as static libraries, even if sdbus-c++ is only a private dependency. 

See also https://runebook.dev/en/docs/cmake/command/export

Author: Krzmbrzl

CMakeLists.txt

@@ -313,6 +313,11 @@ if(SDBUSCPP_INSTALL)
     endif()
 endif()
 
+export(TARGETS ${EXPORT_SET}
+    NAMESPACE SDBusCpp::
+    FILE sdbus-c++-targets
+)
+
 #----------------------------------
 # CPACK
 #----------------------------------