configure: search a sturdy POSIX shell to be used in the testsuite * configure.ac: Add code (partially inspired to checks in gnulib's 'tests/init.sh') to search for a good-enough, not-buggy POSIX/XSI shell to be used in our testsuite. Accordingly AC_SUBSTitute the variable 'AM_TEST_RUNNER_SHELL'. * NEWS: Update. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>