Force serialization of dtrace -G invocations.

[jhatala]

As of ubuntu focal (20.04), systemtap-sdt-dev version 4.2-3ubuntu0.1, the temporary files collide in a build with concurrency (make -j).

The description in the systemtap repo commit 0de9020c9 describes the problem. From what I can tell, that commit introduced the version that we see on focal and it has the problem that we run into in a parallel build of h2o, because as the description mentions:

Note: this still leaves open a race in the case of 2 dtrace processes with identical input/output paths.

dtrace does not take all that long to run, the impact that serializing all dtrace command invocations has on the build time is imperceptible.

The eventual fix in 05c8fa6a5 seems to be a better fix because it looks like it checks for collisions, though I have not tested that version.