genex: Preserve order while evaluating TARGET_OBJECTS

The logic introduced in commit v3.1.0-rc1~688^2~9 (Genex: Evaluate
TARGET_OBJECTS as a normal expression, 2014-02-26) ordered a map
by pointer value and then constructed a list of object files by
iterating over the map.  This is not deterministic.

Since commit v3.1.0-rc1~688^2~5 (cmTarget: Allow any generator
expression in SOURCES property, 2014-03-18) the order produced by the
above-mentioned logic started being used for the actual list of object
files on the link line.  Since it is not deterministic, spurious
re-links occur after re-running CMake simply because the order of
objects changed on the link line.

Fix this by iterating over the original vector of source files instead
of the map.  This has a deterministic order.
genex: Add comments explaining map consistency
Merge topic 'genex-target-objects-ordering' into next

ec9249e1 genex: Add comments explaining map consistency
Merge topic 'genex-target-objects-ordering' into next

caa4b7b8 genex: Preserve order while evaluating TARGET_OBJECTS