Disable backtrace generation in Hurricane::Error constructor.

* Change: In Hurricane::Backtrace constructor, add a boolean parameter
    to enable/disable the actual backtrace generation.
* Change: In Hurricane::Error constructor, disable by default the
    backtrace generation. The backtrace is useful when the Error is
    thrown, and the program therefore stopped. But in many case we
    just issue the error message on the console and try to continue.
    But if the backtrace is enabled, it terribly slow down the
    program.
      Have to think about an clean way to re-enable the trace only
    when the exception is thrown.

hurricane/src/hurricane/Backtrace.cpp

@@ -427,9 +427,11 @@ namespace Hurricane {
 //     3 libstdc++.6.dylib 0x9142514b _ZSt9terminatev + 29
 
 
-  Backtrace::Backtrace ()
+  Backtrace::Backtrace ( bool enabled )
     : _stack()
   {
+    if (not enabled) return;
+    
     if (_inConstructor) {
       _stack.push_back( "[BUG] Backtrace::Backtrace(): An error occurred in the backtace *istself*." );
       _stack.push_back( "" );

hurricane/src/hurricane/Error.cpp

@@ -47,7 +47,7 @@ namespace Hurricane {
     : Exception ()
     , _reason   (reason)
     , _code     (0)
-    , _backtrace()
+    , _backtrace(false)
   { }
 
 
@@ -55,7 +55,7 @@ namespace Hurricane {
     : Exception ()
     , _reason   (reason)
     , _code     (code)
-    , _backtrace()
+    , _backtrace(false)
   { }
 
 
@@ -63,7 +63,7 @@ namespace Hurricane {
     : Exception ()
     , _reason   ()
     , _code     (0)
-    , _backtrace()
+    , _backtrace(false)
   {
     static char     formatted [ 8192 ];
            va_list  args;
@@ -80,7 +80,7 @@ namespace Hurricane {
     : Exception ()
     , _reason   ()
     , _code     (code)
-    , _backtrace()
+    , _backtrace(false)
   {
     static char     formatted [ 8192 ];
            va_list  args;

hurricane/src/hurricane/hurricane/Backtrace.h

@@ -42,7 +42,7 @@ namespace Hurricane {
 
   class Backtrace {
     public:
-                          Backtrace    ();
+                          Backtrace    ( bool enabled );
                          ~Backtrace    ();
       inline std::string  where        () const;
       inline std::string  textWhere    () const;