[Automake-NG] [FYI 2/2] built sources: avoid fork bomb in $(BUILT_SOURCES) handling

Due to how the handling of $(BUILT_SOURCES) was implemented in Automake-NG, a recursive make call in the recipe of any $(BUILT_SOURCES) (or of any of its prerequisites) would have caused an infinite recursion (complete with fork bomb, yuck). Work around the issue. See:

[Automake-NG] [FYI 1/2] coverage: expose fork bomb in $(BUILT_SOURCES) handling

Due to how the handling of $(BUILT_SOURCES) is implemented in Automake-NG, a recursive make call in the recipe of any $(BUILT_SOURCES) (or of any of its prerequisites) will cause an infinite recursion (complete with fork bomb, yuck). Expose the issue in a test case (still failing, of course).