[build] Finer linking flags, some size savings.

This indeed seems to save 1.8 MiB of bytecode! Refs rocq-archive#18.

_tags

@@ -5,20 +5,24 @@
 true: rectypes, thread, package(str), package(unix), package(threads)
 
 # Sertop/Serlib general flags
-true: package(sexplib), package(ppx_sexp_conv)
 true: warn(+1..49-4-44), warn_error(+1..49-42), bin_annot, annot, safe_string
 
+# This needs to go first to avoid flag precedence problems.
+<sertop/sertop_protocol.ml>: package(ppx_sexp_conv)
+
 # Serlib building options
 <serlib/*.{ml,mli}>: coq_lib, coq_clib, coq_kernel, coq_library, coq_intf, coq_engine, coq_pretyping, coq_interp
 <serlib/*.{ml,mli}>: coq_parsing, coq_stm, coq_proofs, coq_ltac
-<serlib/*.{ml,mli}>: package(ppx_import)
+<serlib/*.{ml}>: package(ppx_import), package(ppx_sexp_conv)
+<serlib/*.{ml,mli}>: package(sexplib)
 
 # Sertop building options
 <serlib>: include
 
 <sertop/*.{ml,mli}>: coq_intf
 <sertop/*>: coq_config, coq_lib, coq_clib, coq_kernel, coq_vm, coq_library, coq_engine, coq_pretyping, coq_interp
 <sertop/*>: coq_parsing, coq_printing, coq_tactics, coq_stm, coq_proofs, coq_toplevel, coq_hparsing, coq_ltac
+<sertop/*>: package(sexplib)
 
 # Specifip toplevel
 <sertop/sertop.ml>: package(cmdliner)