c++: provide a means to control how location.hh is included

Users may want to generate the location file elsewhere, say
$top_srcdir/include/ast/location.hh.  Yet, we should not generate
`#include "$top_srcdir/include/ast/location.hh"` but probably
something like `#include <ast/location.hh>`, or `#include
"ast/location.hh", or `#include <location.hh>`.  It entirely depends
on the compiler flags (-I/-isystem) that are used.  Bison cannot guess
what is expected, so let's give the user a means to tell how the
location file should be included.

* data/location.cc (b4_location_file): New.
* data/glr.cc, data/lalr1.cc: Use it.

data/lalr1.cc

@@ -163,8 +163,8 @@ m4_define([b4_shared_declarations],
 # include <vector>
 
 ]b4_cxx_portability[
-]m4_ifdef([b4_location_file],
-          [[# include "]b4_location_file["]])[
+]m4_ifdef([b4_location_include],
+          [[# include ]b4_location_include])[
 ]b4_variant_if([b4_variant_includes])[
 
 ]b4_attribute_define[