REORG: version: move the remaining BUILD_* stuff from haproxy.c to version.c

REORG: version: move the remaining BUILD_* stuff from haproxy.c to version.c

version.c tries to centralize all variables conveying version information,
but there's still an issue with the BUILD_* variables which are only
passed to haproxy.o and are only updated when that one is rebuilt. This
is not very logical given that we can end up with values there which
contradict info from version.c.

Better move all of these to version.c which is systematically rebuilt.
Most of these variables only end up as string concatenation at the
moment. Some of them are even duplicated. In version.c we now have one
variable (or constant) for each of them and haproxy.c references them
in messages. This is much more logical and easier to maintain in a
consistent state.

The patch looks a bit large but it really only moves the ifdefed string
assignment from one file to another, placing them into variables.