rename V variable to lib-version

rename V variable to lib-version

Due to the rise of kbuild type build systems (as the Linux kernel is a
popularly emulated environment), the V variable has become common as a
knob for controlling verbosity.  Unfortunately, if you run `make V=1`
with glibc during install, it fails with weird errors due to the glibc
build already using this variable for versioning information.

Granted, overriding this variable in the glibc context makes no sense
so people shouldn't be doing it, but when paired with build frameworks
that like to use one set of options for all packages, glibc starts to
stick out as an oddball (in that it fails).  Considering it's easy
enough to rename (it's used in just one place), let's do so.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>