build: use this setup.py command because it shows compiler errors tha…

…t might happen.

The `pip install` command only shows errors with -v, and then there's a
ton of other crap to wade through.

tox.ini

@@ -38,6 +38,7 @@ commands =
     python igor.py zip_mods
 
     # Build the C extension and test with the CTracer
+    python setup.py --quiet build_ext --inplace
     python -m pip install -q -e .
     python igor.py test_with_tracer c {posargs}