Simplify admin/unidata Makefile rules

Simplify admin/unidata Makefile rules

* admin/unidata/unidata-gen.el (unidata-gen-files): Use pop.
Also take the output directory as an argument.

* admin/unidata/Makefile.in: Simplify now that unidata-gen-files takes
the output directory as an argument (no need to cd, etc).
(abs_srcdir, abs_builddir): Remove.
(abs_top_builddir): Replace by top_builddir.
(${DSTDIR}/charprop.el): No need to cd.  Pass dest as argument.
(${DSTDIR}/charprop.el, charprop.el):
No need to pass unidata.txt as argument.