Various build system fixes

configure.ac:

* Place a ":" in the action-if-not-found arguments of the
  PKG_CHECK_MODULES() calls for LIBXML and LIBCURL to override the default
  action of quitting with an error, as neither of these libraries are a
  hard requirement

src/Makefile.am:

* Add parsedate.c to BUILT_SOURCES so that it can be handled properly

* Move the parsedate.c rule into the MAINTAINER_MODE block, as only
  maintainers should be regenerating this file

* Bump the "test" rule down after the test-program rules

tests/Makefile.am:

* Add ntriples-2013 to SUBDIRS to fix configure script invocation

tests/turtle/Makefile.am:

* Add logic to prevent breakage when TEST_WARN_FILES is empty
Tru64 V4.0G compatibility fixes