maint: list of names of built programs available in the Makefiles

maint: list of names of built programs available in the Makefiles

This is just a preparatory refactoring in view of future changes.

* configure.ac (AC_SUBST): New 'built_programs'.
* tests/Makefile.am (AM_TESTS_ENVIRONMENT): Simply define the exported
variable 'built_programs' to the expansion of the '$(built_programs)'
AC_SUBST'd make variable.
(.built-programs): Remove this now-unneeded convenience target.
(CLEANFILES, check_DATA): Delete, no longer needed.