message: depends: Propagate well-known vars into depends
  For example, doing:
  make CC=clang CXX=clang++
  Should now propagate these settings down to depends packages
diff: @@ -13,9 +13,18 @@ default_host_OTOOL = $(host_toolchain)otool
  default_host_NM = $(host_toolchain)nm
  define add_host_tool_func
  +ifneq ($(filter $(origin $1),undefined default),)
  +# Do not consider the well-known var $1 if it is undefined or is taking a value
  +# that is predefined by "make" (e.g. the make variable "CC" has a predefined
  +# value of "cc")
  $(host_os)_$1?=$$(default_host_$1)
  $(host_arch)_$(host_os)_$1?=$$($(host_os)_$1)
  $(host_arch)_$(host_os)_$(release_type)_$1?=$$($(host_os)_$1)
  +else
  +$(host_os)_$1=$(or $($1),$($(host_os)_$1),$(default_host_$1))
  +$(host_arch)_$(host_os)_$1=$(or $($1),$($(host_arch)_$(host_os)_$1),$$($(host_os)_$1))
  +$(host_arch)_$(host_os)_$(release_type)_$1=$(or $($1),$($(host_arch)_$(host_os)_$(release_type)_$1),$$($(host_os)_$1))
  +endif
  host_$1=$$($(host_arch)_$(host_os)_$1)
  endef
  