Makefile.am: update bootstrap parser when necessary.

Makefile.am: update bootstrap parser when necessary.

The mechanism of updating bootstrap parser was broken by commit
b2bc24704ebd2a0e6050f755dbd93a5cd987a418 : "Support 'make distcheck'."

Before that commit bootstrap parser was updated every time when autogenerated
parser was updated. This approach contradicted with 'make distcheck' which
mounts top source directory read-only (including bootstrap files). That commit
tried to solve the problem by checking if the autogenerated parser has changed
and only then copying it to bootstrap parser.

I made two mistakes:
    - philisophic mistake: autogenerated parser was actually altered by
      'make distcheck' because of altered paths in '#line' info;
    - shell scripting mistake: the check if the autogenerated parser has
      changed was inverted, so bootstrap parser was never updated;

This commit makes use of two observations:
    - bootstrap parser depends on the source parser, not on the autogenerated
      parser;
    - bootstrap parser shouldn't at all contain line information: at the time
      it is copied to build directory the relative path from build directory
      to source directory may be completely different (as it is with 'make
      distcheck');