[ng] general: assume dot-starting file names are supported

[ng] general: assume dot-starting file names are supported

Now that we've dropped support for DJGPP, Windows 95/98/ME and MS-DOS,
that should always the case.

* lib/am/texibuild.am: Just assume we can have directories whose name
starts with a dot, i.e., that '${am__leading_dot}' expands to '.'.
* m4/depend.m4 (AM_SET_DEPDIR): Likewise.  Now $(DEPDIR) will be set
unconditionally to '.deps'.
* m4/init.m4 (AM_INIT_AUTOMAKE): Don't AC_REQUIRE the m4 macro
'AM_SET_LEADING_DOT' anymore.
* m4/lead-dot.m4: Delete.
* Makefile.am (dist_automake_ac_DATA): Remove it.
* NG-NEWS: Update.
* t/subobj11a.sh: Just assume $(DEPDIR) expands to '.deps'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>