enable_reference_tracking copy constructor does not call base class version

[Paolo]

I get warning messages whenever I use xpressive compiling with gcc -Wextra. The problem is that the copy constructor of enable_reference_tracking does not call that of enable_shared_from_this. To reproduce, just compile the first example in the xpressive manual with the -Wextra flag of gcc.

[Eric Niebler]

In svn head, enable_reference_tracking no longer inherits from enable_shared_from_this. However, I tried compiling xpressive's examples with gcc -Wall and fixed all the warnings.