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.
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