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.
liblzma: Fix invalid Libs.private value in liblzma.pc.
Update NEWS for 5.0.7.
Bump version and soname for 5.0.7.