Add TODO comment for sdk/CMakeLists.txt

It is a short-term workaround for flatcc.

Co-authored-by: Dave Bort <[email protected]>

Author: shewu-quic

sdk/CMakeLists.txt

@@ -108,6 +108,19 @@ add_custom_command(
     ${CMAKE_SOURCE_DIR}/third-party/flatcc/bin/flatcc -cwr -o
     ${_program_schema__include_dir}/executorch/sdk/etdump
     ${_etdump_schema__srcs}
+  # TODO(dbort): flatcc installs its files directly in its source directory
+  # instead of under CMAKE_BINARY_DIR, and it has no options to avoid
+  # doing this. We build flatcc twice in the executorch build: once to get
+  # the `flatcc` host commandline tool, and once to get the (potentially
+  # cross-compiled) target runtime library. The host build will put its outputs
+  # in the source tree, making the cross-compiling target build think that
+  # the outputs have already been built. It will then try to link against the
+  # host-architecture libraries, failing when cross-compiling. To work around
+  # this, delete the host outputs after running this command (which only runs
+  # when setting up the cmake files, not when actually building). This leaves
+  # room for the target build to put its own files in the source tree. We should
+  # try to remove this hack, ideally by submitting an upstream PR that adds an
+  # option to change the installation location.
   COMMAND
     rm -f ${CMAKE_SOURCE_DIR}/third-party/flatcc/bin/*
     ${CMAKE_SOURCE_DIR}/third-party/flatcc/lib/*