multiprocess build cleanup: comment on manual dependencies

Also move manual dependency closer to actual build target

Author: ryanofsky

src/Makefile.am

@@ -330,7 +330,6 @@ obj/build.h: FORCE
 	  "$(abs_top_srcdir)"
 libbitcoin_util_a-clientversion.$(OBJEXT): obj/build.h
 
-ipc/capnp/libbitcoin_ipc_a-protocol.$(OBJEXT): $(libbitcoin_ipc_mpgen_input:=.h)
 
 # server: shared between bitcoind and bitcoin-qt
 # Contains code accessing mempool and chain state that is meant to be separated
@@ -1012,6 +1011,10 @@ libbitcoin_ipc_mpgen_input = \
 EXTRA_DIST += $(libbitcoin_ipc_mpgen_input)
 %.capnp:
 
+# Explicitly list dependencies on generated headers as described in
+# https://www.gnu.org/software/automake/manual/html_node/Built-Sources-Example.html#Recording-Dependencies-manually
+ipc/capnp/libbitcoin_ipc_a-protocol.$(OBJEXT): $(libbitcoin_ipc_mpgen_input:=.h)
+
 if BUILD_MULTIPROCESS
 LIBBITCOIN_IPC=libbitcoin_ipc.a
 libbitcoin_ipc_a_SOURCES = \