[SV 48643] Preserve target-specific variables on intermediate files

[SV 48643] Preserve target-specific variables on intermediate files

Target-specific variables used to define the target as "ought to
exist" so they could never be intermediate.  Now they can be, so
merge the target-specific variables from the intermediate target
so they're not lost.

* src/implicit.c (pattern_search): Use merge_variable_set_lists
to merge target-specific variables.
* tests/scripts/features/implicit_search: Add tests of target-
specific variable assignments and implicit rules.