configure.ac: rework/fix the NEEDS_RESOLV and NEEDS_LIBGEN tests

configure.ac: rework/fix the NEEDS_RESOLV and NEEDS_LIBGEN tests

The "action" parameters for these two tests were supplied incorrectly for
the way the tests were implemented.  The tests check whether a program
which calls hstrerror() or basename() successfully links when -lresolv or
-lgen are used, respectively.  A successful linking would result in
NEEDS_RESOLV or NEEDS_LIBGEN being unset, and failure would result in
setting the respective variable.

Aside from that issue, the tests did not handle the case where neither
library was necessary for accessing the functions in question.  So solve
both of these issues by re-working the two tests so that their form is like
the NEEDS_SOCKET test which attempts to link with just the c library, and
if it fails then assumes that the additional library is necessary and sets
the appropriate variable.

Also an entry in the config.mak.in file is necessary for the NEEDS_LIBGEN
variable to appear in the config.mak.autogen file with the value assigned
by the configure script.  Without it, the generated shell script would
contain a snippet like this:

   for ac_lib in ; do
      ...

which is incorrect.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>