These aren't from autoconf-archive, so don't name them as if they are.
Reset `$ax_lib` before use.

If left unmodified, this leads to confusing error messages as
`$ax_lib` may contain the name of the previous library checked
for, which bypasses the check for any version of the library.

This fixes #18.
Remove manual libprotobuf checks.

We were already checking for protobuf includes and libraries in
the pkg-config check ('DEPS'), so they were redundant. This also
fixes #21.
Merge pull request #24 from MapQuest/configure-fixes

Configure fixes