libphobos: Always build with warning flags enabled

libphobos: Always build with warning flags enabled

This moves WARN_DFLAGS from GDCFLAGS to AM_DFLAGS so it is always
included in the build and testsuite of libphobos.  Currently, this
doesn't happen as GDCFLAGS is overriden by it being set at the
top-level.

libphobos/ChangeLog:

	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Substite WARN_DFLAGS independently of GDCFLAGS.
	* libdruntime/Makefile.am: Add WARN_DFLAGS to AM_DFLAGS.
	* libdruntime/Makefile.in: Regenerate.
	* src/Makefile.am: Add WARN_DFLAGS to AM_DFLAGS.
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/testsuite_flags.in: Add WARN_DFLAGS to --gdcflags.