Replace autotools by cmake for the linux gitian builds, and use
ninja as a generator.
The test_bitcoin binary is no longer part of the gitian output.
Note 2: There are some more dependencies required.
This is because CMake is run for the native platform in order to build a
couple helper binaries for secp256k1 and univalue. CMake is run from the
project root and check for all the dependencies despite only a compiler
is required. This is a minor inconvenience which can be reworked later.