Fix autotools build.

Follows commits 4e002d9a92ecd7cec0fb98b0bedbace8aad81f6e &
7a90710e0d3c786cf4f3af2457a815db6c9b5a5a that only handled CMake.
Fix issue 317

This is the exact same fix as mentionne din the GC issue,
cf. https://code.google.com/p/libarchive/issues/detail?id=317
Make sure has_encrypted_entries is a signed int

Related to cd1740912b18f748a4f9b491a32b44c1712a7478

Trying to assign -1 (ARCHIVE_READ_FORMAT_ENCRYPTION_DONT_KNOW)
to a data type we're not sure will always be signed is a bad idea
(blew up on ARM, for instance).
Missed two tests!

(Mentioned in #58, thanks!)
Merge pull request #53 from NiLuJe/fix-issue-317

Fix issue 317
Merge pull request #54 from NiLuJe/fix-autotools-build

Fix autotools build.
Merge pull request #57 from NiLuJe/fix-has_encrypted_entries

Make sure has_encrypted_entries is a signed int