Fix memmem test for mingw. * modules/memmem-tests (configure.ac): Check for alarm. * tests/test-memmem.c (main): Avoid alarm on platforms that lack it. * doc/functions/memmem.texi: New file. * doc/gnulib.texi (Function Substitutes): Add memmem. Reported by Bruno Haible. Signed-off-by: Eric Blake <ebb9@byu.net>