ports/57698: Add USE_APACHE knob to bsd.port.mk.

[Clement Laforet]

> Don't use single number version numbers (2), but use 20 or 21 (yes
> there is a repocopy pending for apache21)

And apr versions are different too :-)
Some module will support apache 2.0.x and 2.1/2.2 and some won't.
In fact you should revival PR ports/18960 and add few things.
A real ckeck (i.e. get rid of .exists detection) should be considered I
think.

regards,
clem