adding "make valgrind" running the full regression tests (except python

* Makefile.am: adding "make valgrind" running the full regression
  tests (except python ones) under Valgrind (using valgrind -q
  which was kindly added by the author).
* valid.c: stupid bug pinpointed by Valgrind, the regression tests
  passes cleanly now except an obcure floating point initialization
  raised in log10() in one XPath regression test ???
* tree.c: edited some comments to close #75244
Daniel
4 files changed