configure: Fix --enable-gtk=auto when building for OS X

The default value for the --enable-gtk option is ‘auto’. When building
for OS X it was supposed to detect this and change it to ‘no’. However
it was missing this step and leaving it as auto which was causing the
configure script to fail later.
Use and verify a signature on the automatic updates on OS X

In the tools directory a tool called rig-check-signature now gets
built. This uses libcrypto to verify the downloaded update against a
known public key. The public key is stored in the git repo. The
tarballs for the update are now prefixed with a 128 byte signature
which signs the SHA256 digest of the remainder of the file. If the
signature matches the tool exits with 0 status and writes the
remainder of the file out to a given location.

The automatic update script on OS X is now configured to use the tool
before moving the extracted data to the update filename. That way it
should only use trusted files.

The build scripts for OS X now also build a static library of
libcrypto so that it can be linked to by the tool.