cmake: Support building with libmultiprocess subtree

When ENABLE_IPC option is on, build with libmultiprocess subtree and
`add_subdirectory(src/ipc/libmultiprocess)` instead of external package
and `find_package(Libmultiprocess)` by default.

Behavior can be toggled with `WITH_EXTERNAL_LIBMULTIPROCESS` option. Using a
subtree should be more convenient for most bitcoin developers, but using an
external package is more convenient for developing in the libmultiprocess
repository.

The `WITH_EXTERNAL_LIBMULTIPROCESS` option is also used to avoid needing to
changing the depends build here. But in later commits, the depends build is
switched to use the add_subdirectory build as well.

Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>
Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

CMakeLists.txt

@@ -154,7 +154,8 @@ endif()
 cmake_dependent_option(WITH_DBUS "Enable DBus support." ON "CMAKE_SYSTEM_NAME STREQUAL \"Linux\" AND BUILD_GUI" OFF)
 
 option(ENABLE_IPC "Build multiprocess bitcoin-node and bitcoin-gui executables in addition to monolithic bitcoind and bitcoin-qt executables. Requires libmultiprocess library. Experimental." OFF)
-if(ENABLE_IPC)
+cmake_dependent_option(WITH_EXTERNAL_LIBMULTIPROCESS "Build with external libmultiprocess library instead of with local git subtree when ENABLE_IPC is enabled. This is not normally recommended, but can be useful when cross-compiling or making changes to the upstream project." OFF "ENABLE_IPC" OFF)
+if(ENABLE_IPC AND WITH_EXTERNAL_LIBMULTIPROCESS)
   find_package(Libmultiprocess REQUIRED COMPONENTS Lib)
   find_package(LibmultiprocessNative REQUIRED COMPONENTS Bin
     NAMES Libmultiprocess
@@ -674,6 +675,16 @@ if(ENABLE_WALLET)
 endif()
 message("  external signer ..................... ${ENABLE_EXTERNAL_SIGNER}")
 message("  ZeroMQ .............................. ${WITH_ZMQ}")
+if(ENABLE_IPC)
+  if (WITH_EXTERNAL_LIBMULTIPROCESS)
+    set(ipc_status "ON (with external libmultiprocess)")
+  else()
+    set(ipc_status ON)
+  endif()
+else()
+  set(ipc_status OFF)
+endif()
+message("  IPC ................................. ${ipc_status}")
 message("  USDT tracing ........................ ${WITH_USDT}")
 message("  QR code (GUI) ....................... ${WITH_QRENCODE}")
 message("  DBus (GUI, Linux only) .............. ${WITH_DBUS}")