travis: build sources with tracing enabled

Our tracing architecture is not built by default, causing the Travis CI
to not execute some code and skip several tests. As AppVeyor has already
enabled the tracing architecture when building the code, we should do
the same for Travis CI to have this code being tested on macOS and
Linux.

Add "-DENABLE_TRACE=ON" to our release-build options of Travis.

.travis.yml

@@ -17,7 +17,7 @@ env:
   - secure: "YnhS+8n6B+uoyaYfaJ3Lei7cSJqHDPiKJCKFIF2c87YDfmCvAJke8QtE7IzjYDs7UFkTCM4ox+ph2bERUrxZbSCyEkHdjIZpKuMJfYWja/jgMqTMxdyOH9y8JLFbZsSXDIXDwqBlC6vVyl1fP90M35wuWcNTs6tctfVWVofEFbs="
   - GITTEST_INVASIVE_FS_SIZE=1
  matrix:
-  - OPTIONS="-DTHREADSAFE=ON -DCMAKE_BUILD_TYPE=Release -DENABLE_WERROR=ON"
+  - OPTIONS="-DTHREADSAFE=ON -DENABLE_TRACE=ON -DCMAKE_BUILD_TYPE=Release -DENABLE_WERROR=ON"
   - OPTIONS="-DTHREADSAFE=OFF -DBUILD_EXAMPLES=ON -DENABLE_WERROR=ON"
 
 dist: trusty