python.port.mk: Respect SEPARATE_BUILD in MODPY_DISTUTILS_BUILD

Out-of-tree builds for simple python ports are currently built in WRKSRC
even when SEPARATE_BUILD is set.

Simple example:

$ cd /usr/ports/www/linkchecker
$ make build
[...]
$ make show='SEPARATE_BUILD WRKSRC WRKBUILD'
SEPARATE_BUILD=Yes
WRKSRC=/usr/obj/ports/linkchecker-9.3/linkchecker-upstream-9.3
WRKBUILD=/usr/obj/ports/linkchecker-9.3/build-amd64
$ ls `make show=WRKBUILD`
$

That's because MODPY_DISTUTILS_BUILD hardcodes WRKSRC, the following
diff changes that to WRKBUILD which defaults to WRKSRC if SEPARATE_BUILD
set to "no".

Tested successfully with ports like www/linkchecker and sysutils/ranger.

Feedback?

Index: lang/python/python.port.mk
===================================================================
RCS file: /cvs/ports/lang/python/python.port.mk,v
retrieving revision 1.95
diff -u -p -r1.95 python.port.mk
--- lang/python/python.port.mk 8 Jul 2017 08:28:00 -0000 1.95
+++ lang/python/python.port.mk 25 Jan 2018 21:49:23 -0000
@@ -128,7 +128,7 @@ MODPY_SITEPKG = ${MODPY_LIBDIR}/site-pa
MODPY_SETUP ?= setup.py

# build or build_ext are commonly used
-MODPY_DISTUTILS_BUILD ?= build --build-base=${WRKSRC}
+MODPY_DISTUTILS_BUILD ?= build --build-base=${WRKBUILD}

.if ${MODPY_SETUPUTILS:L} == "yes"
MODPY_DISTUTILS_INSTALL ?= install --prefix=${LOCALBASE} \