[SV 65006] Allow secondary expansion of .EXTRA_PREREQS

* src/rule.c (snap_implicit_rules): Set need_2nd_expansion of each
  prerequisite of global .EXTRA_PREREQS.
* src/file.c (snap_file):  Set need_2nd_expansion of each prerequisite
  of target-specific .EXTRA_PREREQS.
* tests/scripts/variables/EXTRA_PREREQS: Add tests.

Reported by Daniel Gerber <dg@atufi.org>.