Fixed typo in xmlCharEncFirstLine pointed out by Mark Rowe (bug #440159)

* encoding.c: Fixed typo in xmlCharEncFirstLine pointed out
  by Mark Rowe (bug #440159)
* include/libxml/xmlversion.h.in: Added check for definition of
  _POSIX_C_SOURCE to avoid warnings on Apple OS/X (patch from
  Wendy Doyle and Mark Rowe, bug #346675)
* schematron.c, testapi.c, tree.c, xmlIO.c, xmlsave.c: minor
  changes to fix compilation warnings - no change to logic.

svn path=/trunk/; revision=3618