* aclocal.in (@file_order): New variable, to make sure

* aclocal.in (@file_order): New variable, to make sure
files are output in the opposite order of the -I arguments.
(scan_file): Fill it.
(trace_used_macros, write_aclocal): Use it.
(scan_m4_files): Reverse the directory contents, so that
macros from the lexicographically greatest files are preferred.
* tests/aclocal9.test, tests/acloca10.test, tests/acloca11.test:
New files.
* tests/Makefile.am (TESTS): Add them.