Update TODO.
Update NEWS for 5.0.6 and 5.1.4beta.
Bump the version number to 5.1.4beta.
liblzma: Fix a portability problem in Makefile.am.

POSIX supports $< only in inference rules (suffix rules).
Using it elsewhere is a GNU make extension and doesn't
work e.g. with OpenBSD make.

Thanks to Christian Weisgerber for the patch.
Update NEWS for 5.0.7.
l10n: vi.po: Update Vietnamese translation

Signed-off-by: Trần Ngọc Quân <vnwildman@gmail.com>