_MYSQL_VERSION and supporting multiple mysql versions

[David Brownlee <abs%absd.org@localhost>]

I've seen (and am quitre possibly about to copy) the following idiom a
few times:

.include "../../mk/mysql.buildlink3.mk"
.if ${_MYSQL_VERSION} == "55"
DEPENDS+=       mysql-server-[0-9]*:../../databases/mysql55-server
.elif ${_MYSQL_VERSION} == "51"
DEPENDS+=       mysql-server-[0-9]*:../../databases/mysql51-server
.else
DEPENDS+=       mysql-server-[0-9]*:../../databases/mysql5-server
.endif

Should this be abstracted out, similar to way version of packages such
as python are handled?