Adding options framework variables that depend on variables coming after bsd.port.pre.mk

[Naram Qashat]

I'm wanting to add an options framework LIB_DEPENDS that depends on
variables that normally come after bsd.port.pre.mk.

In this case, I am trying to add a UTEMPTER_LIB_DEPENDS on
sysutils/libutempter, but I want to check OPSYS and OSVERSION beforehand,
since base FreeBSD 9.x contains utempter while any earlier FreeBSDs as
well as non-FreeBSD OSes don't contain it and need the port.

I cannot place the UTEMPTER_LIB_DEPENDS line after bsd.port.pre.mk because
the options framework doesn't see it then, but I also cannot wrap it
around an if conditional using OPSYS and OSVERSION before bsd.port.pre.mk
because those variables aren't set yet.

I've been told that there is a workaround, but I was not told what the
workaround was. Does anyone know how this can be acomplished?