Don't impose std::string for filenames.

Don't impose std::string for filenames.
* data/lalr1.cc (b4_filename_type): New.
(position::filename): Use it.
(parser.hh): Move the inclusion of stack.hh and location.hh below
the user code, so that needed headers for the filename type can be
included first.
Forward declare them before the user code.
* tests/Makefile.am (check-local, installcheck-local): Pass
TESTSUITEFLAGS to the TESTSUITE.