build: add a dependency on the gnulib tempname module

build: add a dependency on the gnulib tempname module

* bootstrap.conf: Add "tempname" which is needed by mktemp(1).
The explicit dependency supports running gnulib-tool with
the --conditional-dependencies option, used to minimize built
modules.  Note on a Fedora 22 system, that results in avoiding
redundant builds of: areadlinkat.o asnprintf.o fd-hook.o
fseterr.o printf-args.o printf-parse.o sockets.o vasnprintf.o.
However --conditional-dependencies is not enabled, since it
currently precludes the inclusion of gnulib tests.