maint: make position of gnulib checkout configurable

maint: make position of gnulib checkout configurable

Some gnulib-related tools (most prominently, the gnulib-provided
'bootstrap' script) allow the user to define the position of his
gnulib's repository checkout through the use of the 'GNULIB_SRCDIR'
environment variable.  We should do the same, for consistency and
to easily support slightly unusual layouts in developers' source
trees.

* cfg.mk (gnulib_dir): Define to "$GNULIB_SRCDIR" if that's set,
and to default value of "'$(abs_srcdir)'/../gnulib" otherwise.
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