Add comparison operators for C++ location classes. Discussed at

Add comparison operators for C++ location classes.  Discussed at
<http://lists.gnu.org/archive/html/bison-patches/2006-06/msg00092.html>.
* data/c++.m4 (b4_define_location_comparison): New boolean %define
declaration indicating whether filename_type has an operator==.  If
filename_type is `std::string', it defaults to `1', `0' otherwise.
* data/location.cc: Iff b4_define_location_comparison is `1', add
operator== and operator!= for class position and for class location.

Some minor fixes.
* src/scan-action.l: Remove unused file.
* src/symtab.c (symbol_printer_set): Use printer_location not
destructor_location.
* src/symtab.h (struct symbol): Replace incorrect source comment for
printer members.
* tests/input.at (Incompatible Aliases): Update output with correct
printer location.