Our performance tests (or to be more concrete, our single performance
test) are not built by default, as they are always #ifdef'd out. While
it is true that we don't want to run performance tests by default, not
compiling them at all may cause code rot and is thus an unfavorable
approach to handle this.

We can easily improve this situation: this commit removes the #ifdef,
causing the code to always be compiled. Furthermore, we add `-xperf` to
the default command line parameters of `generate.py`, thus causing the
tests to be excluded by default.

Due to this approach, we are now able to execute the performance tests
by passing `-sperf` to `libgit2_clar`. Unfortunately, we cannot execute
the performance tests on Travis or AppVeyor as they rely on history
being available for the libgit2 repository. As both do a shallow clone
only, though, this is not given.

(cherry picked from commit 543ec149)