Restore 8954fd436c7 after c06a8f9caa51c

Else, just-built clang can't build programs that include libc++ headers
on macOS if you build via the 'all' target.

GitOrigin-RevId: d3b74dc1e472c773c27dbbe2b3ae2a3cd0b68c17
diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 4c7fd86..c044a5d 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -204,7 +204,7 @@
   list(APPEND _all_includes "${dst}")
 endforeach()
 
-add_custom_target(generate-cxx-headers DEPENDS ${_all_includes})
+add_custom_target(generate-cxx-headers ALL DEPENDS ${_all_includes})
 
 add_library(cxx-headers INTERFACE)
 add_dependencies(cxx-headers generate-cxx-headers ${LIBCXX_CXX_ABI_HEADER_TARGET})