build: depends makes libmultiprocess by default

This causes IPC binaries (bitcoin-node, bitcoin-gui) to be included
in releases.

The effect on CI is that this causes more depends builds to build IPC
binaries, but still the only build running functional tests with them
is the i686_multiprocess one.

Except for Windows.

depends/toolchain.cmake.in

@@ -165,11 +165,12 @@ else()
   set(WITH_USDT ON CACHE BOOL "")
 endif()
 
-if("@multiprocess@" STREQUAL "1")
+set(ipc_packages @ipc_packages@)
+if("${ipc_packages}" STREQUAL "")
+  set(ENABLE_IPC OFF CACHE BOOL "")
+else()
   set(ENABLE_IPC ON CACHE BOOL "")
   set(MPGEN_EXECUTABLE "${CMAKE_CURRENT_LIST_DIR}/native/bin/mpgen" CACHE FILEPATH "")
   set(CAPNP_EXECUTABLE "${CMAKE_CURRENT_LIST_DIR}/native/bin/capnp" CACHE FILEPATH "")
   set(CAPNPC_CXX_EXECUTABLE "${CMAKE_CURRENT_LIST_DIR}/native/bin/capnpc-c++" CACHE FILEPATH "")
-else()
-  set(ENABLE_IPC OFF CACHE BOOL "")
 endif()