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