depends: Allow building with system clang

depends/Makefile

@@ -156,7 +156,11 @@ $(host_arch)_$(host_os)_native_toolchain?=$($(host_os)_native_toolchain)
 include funcs.mk
 
 binutils_path=$($($(host_arch)_$(host_os)_native_binutils)_prefixbin)
+ifeq ($(strip $(FORCE_USE_SYSTEM_CLANG)),)
 toolchain_path=$($($(host_arch)_$(host_os)_native_toolchain)_prefixbin)
+else
+toolchain_path=
+endif
 final_build_id_long+=$(shell $(build_SHA256SUM) config.site.in)
 final_build_id+=$(shell echo -n "$(final_build_id_long)" | $(build_SHA256SUM) | cut -c-$(HASH_LENGTH))
 $(host_prefix)/.stamp_$(final_build_id): $(native_packages) $(packages)