Warn on unused return values in guidance code, resolves #2686.

https://github.com/Project-OSRM/osrm-backend/pull/2685/files

fixes an issue where we did

    elongate(fstStep, sndStep);

instead of

    newStep = elongate(fstStep, sndStep);

we didn't get any warnings.

The only way to trigger a warning here is to use

```cpp
__attribute__((warn_unused_result))
```

This changeset does exactly that: for the new guidance code prone to
these kind of issue we add such an attribute to the declaration.

src/engine/guidance/post_processing.cpp

@@ -5,6 +5,7 @@
 #include "engine/guidance/lane_processing.hpp"
 #include "engine/guidance/toolkit.hpp"
 
+#include "util/attributes.hpp"
 #include "util/guidance/toolkit.hpp"
 #include "util/guidance/turn_lanes.hpp"
 
@@ -127,6 +128,7 @@ double turn_angle(const double entry_bearing, const double exit_bearing)
     return angle > 360 ? angle - 360 : angle;
 }
 
+OSRM_ATTR_WARN_UNUSED
 RouteStep forwardInto(RouteStep destination, const RouteStep &source)
 {
     // Merge a turn into a silent turn
@@ -325,6 +327,7 @@ void closeOffRoundabout(const bool on_roundabout,
 }
 
 // elongate a step by another. the data is added either at the front, or the back
+OSRM_ATTR_WARN_UNUSED
 RouteStep elongate(RouteStep step, const RouteStep &by_step)
 {
     BOOST_ASSERT(step.mode == by_step.mode);