Correct library handling in autoconf

Ensure that libresolv is detected when it is really needed, and move
libresolv and libidn out of LIBS and into SUPPORTING_LIBS so they can
be included in just libhesiod.
Install a pkg-config file for hesiod
Add more gitignores.

We now have a hesiod.pc to ignore, and apparently some newer autoconfs
start giving us a test-driver.