Add CMAKE_BUILD_INTERFACE_INCLUDES build-variable.

This makes

 set(CMAKE_BUILD_INTERFACE_INCLUDES ON)

add the equivalent of

 set_property(TARGET tgt APPEND PROPERTY
   INTERFACE_INCLUDE_DIRECTORIES
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR};${CMAKE_CURRENT_BINARY_DIR}>
 )

to every target.

If the headers are in CMAKE_CURRENT_SOURCE_DIR, and the generated headers
are in CMAKE_CURRENT_BINARY_DIR, this is a convenient way to build a target
bar, which depends on foo, just by using target_link_libraries() and adding
the INTERFACE_INCLUDE_DIRECTORIES to the INCLUDE_DIRECTORIES of the target
being linked. There will be more-convenient porcelain API to consume the
property in the future.
Merge topic 'include-dirs-convenience' into next

9ce1b9e Add CMAKE_BUILD_INTERFACE_INCLUDES build-variable.