depends: reuse _config_opts for CMake options

This will allow us to use the existing machinery for filtering by
arch, os, debug/release, etc.

For example, the following becomes possible for libevent:

$(package)_config_opts_release=-DEVENT__DISABLE_DEBUG_MODE

Now the define is only set when not building depends with DEBUG=1

depends/packages/libmultiprocess.mk

@@ -10,8 +10,8 @@ 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++"
+$(package)_config_opts := -DCAPNP_EXECUTABLE="$$(native_capnp_prefixbin)/capnp"
+$(package)_config_opts += -DCAPNPC_CXX_EXECUTABLE="$$(native_capnp_prefixbin)/capnpc-c++"
 endif
 endef