Support 'make distcheck'.

Support 'make distcheck'.

The only problem with 'make distcheck' was that we needed write
access to top source directory ('make' wanted to overwrite bootstrap
parser if it was built with bison and 'make check' wanted to create
temporary files in 'test/' directory).

This commit fixes it:
    - 'make' doesn't try to overwrite bootstrap if it is identical
      to the existing one (must always be true for 'make distcheck')
    - testing script makes a temporary directory and keeps all
      temporary files there. If some tests failed, temporary files
      for them are left and test sources and reference results are
      copied into temporary directory to make debug more convenient.

This commit makes use of 'make distcheck' in release script.