bsd.python.mk broken

[Dominic Fandrey]

If one uses the OPTIONS framework like in graphics/opencv
to trigger python support, bsd.python.mk breaks. Here is why:

USE_PYTHON has to be set before including bsd.port.pre.mk

Unfortunately it can only be set after bsd.port.pre.mk, because
WITH_PYTHON wouldn't be set before.

For this the _POSTMKINCLUDED variable is responsible. The
following is from the beginning of bsd.python.mk:#
.if !defined(_POSTMKINCLUDED) && !defined(Python_Pre_Include)

It looks similar for bsd.perl.mk, so maybe the problem
exists there as well. bsd.php.mk or bsd.java.mk are different:
.if !defined(Java_Include)

I suggest removing the !defined(_POSTMKINCLUDED) from both
bsd.python.mk and bsd.perl.mk.