maint: use xconcat-filename.

maint: use xconcat-filename.

* bootstrap.conf (gnulib_modules): Request it.
* src/output.h, src/output.c (compute_pkgdatadir): Rename as...
(pkgdatadir): this.
Adjust dependencies.
* src/output.c (output_skeleton): Reduce the scope of "in".
Use xconcatenated_filename to simplify the construction of the
qualified paths to m4sugar.m4, bison.m4, and the selected skeleton.
There are a few minor differences: the new code uses strchr instead of
mbschr (but this was not really justified), and the new code does not
garantee a single slash even if $BISON_PKGDATADIR ends with several
(which was considered more accurate).  See the discussion at
<http://lists.gnu.org/archive/html/bison-patches/2012-05/msg00052.html>.