Genex: Fix the HEAD target used for evaluated expressions.

If the expression $<TARGET_PROPERTY:prop> appears in the content
of a target property, the target that prop is read from is
the 'head target' of the expression. In contexts such as evaluating
the content of a target property during generation, such
as INCLUDE_DIRECTORIES, the 'head target' is the one on which the
initial request was made.

If evaluating a generator expression which is not a target property
content, the target must be explicitly specified. Such contexts
include add_custom_command and file(GENERATE). The content might
then look like

 $<TARGET_PROPERTY:tgt,prop>

However, as there is no HeadTarget set, any generator expressions
evaluated as part of reading prop from tgt which do not specify
the tgt directly report an error.

Modify the logic of the TARGET_PROPERTY generator expression so
that in such contexts, the 'head target' is set to the appropriate
target which was first encountered.
Merge topic 'fix-genex-HEAD-target' into next

acf2317 Genex: Fix the HEAD target used for evaluated expressions.