If OBJEXT and EXEEXT are not set, provide a default value, and use

If OBJEXT and EXEEXT are not set, provide a default value, and use
them unconditionally.
* automake.in (&generate_makefile): Provide default values for
EXEEXT and OBJEXT.
(&get_object_extension, &finish_languages)
(&handle_single_transform_list, &handle_programs, &add_depend2)
(&check_cygnus, &lang_c_finish, am_install_var): Don't bother with
them, they are defined.
* compile.am, depend2.am, program.am, progs.am: Likewise.
* remake-hdr.am: Fix a typo.
* tests/cxxo.test, tests/fo.test, tests/implicit.test,
* tests/interp.test, tests/subobj.test, tests/subobj2.test,
* tests/suffix2.test: Adjust.