build: Fix libmultiprocess cross-compiling to Linux hosts

To successfully call the `capnp_generate_cpp()` function, the
`libmultiprocess` build system must be provided with paths to the native
`capnp` and `capnpc-c++` tools.

depends/funcs.mk

@@ -175,7 +175,7 @@ $(1)_cmake=env CC="$$($(1)_cc)" \
                CXX="$$($(1)_cxx)" \
                CXXFLAGS="$$($(1)_cppflags) $$($(1)_cxxflags)" \
                LDFLAGS="$$($(1)_ldflags)" \
-             cmake -DCMAKE_INSTALL_PREFIX:PATH="$$($($(1)_type)_prefix)"
+             cmake -DCMAKE_INSTALL_PREFIX:PATH="$$($($(1)_type)_prefix)" $$($(1)_cmake_opts)
 ifeq ($($(1)_type),build)
 $(1)_cmake += -DCMAKE_INSTALL_RPATH:PATH="$$($($(1)_type)_prefix)/lib"
 else

depends/packages/libmultiprocess.mk

@@ -4,6 +4,16 @@ $(package)_download_path=$(native_$(package)_download_path)
 $(package)_file_name=$(native_$(package)_file_name)
 $(package)_sha256_hash=$(native_$(package)_sha256_hash)
 $(package)_dependencies=native_$(package) capnp
+ifneq ($(host),$(build))
+$(package)_dependencies += native_capnp
+endif
+
+define $(package)_set_vars :=
+ifneq ($(host),$(build))
+$(package)_cmake_opts := -DCAPNP_EXECUTABLE="$$(native_capnp_prefixbin)/capnp"
+$(package)_cmake_opts += -DCAPNPC_CXX_EXECUTABLE="$$(native_capnp_prefixbin)/capnpc-c++"
+endif
+endef
 
 define $(package)_config_cmds
   $($(package)_cmake) .