Merge #25214: multiprocess build fix: ipc/capnp/init.capnp.h: No such file or directory

44904aa multiprocess build cleanup: comment on manual dependencies (Ryan Ofsky)
6e1c16c multiprocess build fix: ipc/capnp/init.capnp.h: No such file or directory (Ryan Ofsky)

Pull request description:

  Error was reported by SatoriHoshiAiko in #25207 and happens unpredictably because make doesn't always build dependencies in the same order.

  The source file `src/ipc/capnp/protocol.cpp` includes some generated headers so needs to have an explicit dependency specified in the makefile so the headers will be generated before the file is compiled. #19160 added the explicit dependency, but it was incorrect because it referred to an old file path from before the source file was renamed (`ipc.cpp` -> `protocol.cpp`)

ACKs for top commit:
  hebasto:
    re-ACK 44904aa

Tree-SHA512: 578ef4ef35a97e9d8d4e6d4edf39e57a32674234bf145e8159268324cb6ba15b420517afc07f6f42bb11a562954ea74af686c3fb92ce178c1fc378421b352531

Author: fanquake

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-ipc.$(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 = \