Modernize and improve test scripts `subdir*.test'. * tests/subdir.test: Enable `errexit' shell flag, and related changes. Use the `configure.in' stub created by `./defs', rather than writing one from scratch. * tests/subdir2.test: Likewise. * tests/subdir4.test: Likewise. * tests/subdir3.test: Enable `errexit' shell flag, and related changes. Do not create useless dummy source files. * tests/subdir5.test: Make grepping of Makefile.in (in topdir and in subdirs) stricter. Some minor changes to keep it more in sync with the related test `subdir8.test'. * tests/subdir8.test: Likewise (but with the related test being `subdir5.test' here). * tests/subdir6.test: Cosmetic change in spacing. * tests/subdir9.test: Define and use new variable `$distdir'. Add trailing `:' command. * tests/subdir10.test: Cosmetic consistency-related change. * tests/subdirbuiltsources.test: Cosmetic changes in spacings. (configure.in): Use stub created by `./defs', rather than writing it from scratch. Do not use obsoleted and/or deprecated forms of autoconf/automake macros.