Use `_dirstamp' when the file-system doesn't support `.dirstamp'.

Use `_dirstamp' when the file-system doesn't support `.dirstamp'.
* m4/lead-dot.m4: New file.
* m4/Makefile.am (dist_m4data_DATA): Add lead-dot.m4
* m4/init.m4 (AM_INIT_AUTOMAKE): Require AM_SET_LEADING_DOT.:
* m4/depend.m4 (AM_SET_DEPDIR): Require AM_SET_LEADING_DOT. Define
DEPDIR using am__leading_dot.
* automake.in (require_build_directory): Define am__dirstamp, a new
variable for the dirstamp basename, based on am__leading_dot.
* tests/subobj6.test: Move all `test -f' into Makefiles, so we can
use $(am__dirstamp).
Reported by Richard Dawe.