fix: Preserve folder structure of installed proto header files.

The mechanism using the `PUBLIC_HEADERS` properties does not seem to
preserve the folder structure of the header files provided to it.
Consequently, `extensions/extensions.pb.h` ended up on the same level as
the other header files. This commit uses a different mechanism based on
`FILE_SET`, which, however, is only available since CMake v2.23.

src/substrait/proto/CMakeLists.txt

@@ -79,7 +79,10 @@ endforeach()
 # Add the generated protobuf C++ files to our exported library.
 add_library(substrait_proto ${PROTO_SRCS} ${PROTO_HDRS} ProtoUtils.cpp
                             ProtoUtils.h)
-set_target_properties(substrait_proto PROPERTIES PUBLIC_HEADER "${PROTO_HDRS}")
+target_sources(
+  substrait_proto PUBLIC FILE_SET HEADERS BASE_DIRS
+                         ${PROTO_OUTPUT_TOPLEVEL_DIR} FILES ${PROTO_HDRS})
+# set_target_properties(substrait_proto PROPERTIES PUBLIC_HEADER "${PROTO_HDRS}")
 
 # Include the protobuf library as a dependency to use this class.
 target_link_libraries(substrait_proto protobuf::libprotobuf)
@@ -96,4 +99,4 @@ install(
   TARGETS substrait_proto
   EXPORT SubstraitTargets
   LIBRARY
-  PUBLIC_HEADER DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/substrait/proto")
+  FILE_SET HEADERS DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/substrait/proto")