Autoconf, Fix AM_CONFIG_HEADER, AM_PROG_CC_STDC and INCLUDES

Since automake 1.13, the AM_CONFIG_HEADER is obsolete and will fail unless is
it replaced with AC_CONFIG_HEADERS.
minor, fixed a warning about rememberlevel param
minor copyright update.
fixed a bug which makes the administration menu missing

When gcompris --reread-menu was run, one time on 2
the administration menu was added, the other time it
was removed.
advanced color, minor documentation update.
gletters, speed takes in account the global GCompris level speed configuration.
Added missing digraphs
Some unfinished gettext domain fiddling. Need to commit so I can merge upstream
Merge remote-tracking branch 'upstream/master' + Bugfix gletters

Conflicts:
	src/gcompris/menu.c
	src/gletters-activity/gletters.c
	src/gletters-activity/resources/gletters/default-gd.xml

Reinstated customizable speed in gletters.c - setSpeed()
Updated the Gaelic XML files in gletters resources to slow down