timer: add a stopwatch for timing a simple operation

* src/misc/timer.hh (stopwatch): New class, implemented on top
of C++11's std::chrono::high_resolution_clock.
* src/bin/dstar2tgba.cc, src/bin/ltl2tgba.cc, src/bin/ltlcross.cc:
Use it in lieu of gethrxtime(), so we do not need to distribute
gethrxtime anymore.

src/misc/timer.hh

@@ -29,6 +29,7 @@
 # include <iosfwd>
 # include <string>
 # include <map>
+# include <chrono>
 # if SPOT_HAVE_SYS_TIMES_H
 #  include <sys/times.h>
 # endif
@@ -40,6 +41,33 @@ namespace spot
   /// \addtogroup misc_tools
   /// @{
 
+
+  /// \brief A simple stopwatch
+  struct stopwatch
+  {
+  protected:
+    typedef std::chrono::high_resolution_clock clock;
+    clock::time_point start_;
+  public:
+    /// Marks the start if the measurement
+    void start()
+    {
+      start_ = clock::now();
+    }
+
+    /// \brief Returns the elapsed duration in seconds.
+    ///
+    /// May be called multiple times, and will always return the
+    /// duration since the last call to start().
+    double stop()
+    {
+      auto t = clock::now();
+      typedef std::chrono::duration<double> seconds;
+      return std::chrono::duration_cast<seconds>(t - start_).count();
+    }
+  };
+
+
   /// A structure to record elapsed time in clock ticks.
   struct time_info
   {