In preparation for adding a --disable-giac option, we add a new
feature that detects the presence of the "giac" executable. We already
have a feature for sage.libs.giac, but that only guards the libgiac
interface; we still have code that runs "giac" behind pexpect. This
will allow us to skip those tests when giac is not installed.

Various implementations get it a little (1e-10) different.

…wers

We have an integration test in this file that is looking for the giac
result, but sympy gives an equivalent one. We should accept that, too.

There's one piecewise test that uses algorithm='giac', but it will
soon be possible to install sage without giac. We could make the test
conditional on giac's presence, but the same thing works with sympy,
so let's just use that.