Fix the GCC build (#40)

* Make it compile with GCC.  
* Add GCC to CI.
* Add warning to documentation about using GCC.

src/test/measuretime.h

@@ -11,4 +11,4 @@
     auto diff__ = finish__ - start__; \
     std::cout << name << ": " << std::setw(12) << diff__.count() << " ns" \
               << std::endl; \
-  }
+  }