Tracing is not enabled in debug builds

[BrunoDutertre]

In a debug build:

cmake .. -DCMAKE_BUILD_TYPE=Debug
make VERBOSE=1

I see these compiler options:

-DETHOS_ASSERTIONS -DETHOS_DEBUG -DETHOS_TRACING 

But they have no effect. The source code uses EO_TRACING and EO_ASSERTION instead. and there's no
DEBUG anywhere in the source.

I'm on commit 9b2fd9b.

[ajreynol]

This is fixed by #62.