The THIS_FILE macro currently always gets an empty value. Looking at the definition of SetupNativeCompilation, we have:

$1_THIS_FILE = -DTHIS_FILE='"$$(<F)"'

Which looks ok, but further down:

$1_FLAGS := $(CFLAGS_CCACHE) $$($1_USE_PCH_FLAGS) $$($1_BASE_CFLAGS) \
          $$($1_OPT_CFLAGS) $$($1_CFLAGS) $$($1_THIS_FILE) -c

The $1_FLAGS is assigned using :=, forcing the $1_THIS_FILE variable to evaluate there instead of later in the recipe.