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/glr.cc

@@ -247,7 +247,7 @@ b4_percent_code_get([[requires]])[
 #include <string>
 
 ]m4_ifdef([b4_location_file],
-          [[# include "]b4_location_file["]])[
+          [[# include ]b4_location_include])[
 
 ]b4_null_define[