PORTS_MODULES breakage on HEAD

Adding PORTS_MODULES=emulators/virtualbox-ose-kmod recently broke on
HEAD.  When I do that I get this failure:

===> Ports module emulators/virtualbox-ose-kmod (all)
cd ${PORTSDIR:-/usr/ports}/emulators/virtualbox-ose-kmod; PATH=/usr/obj/usr/src/
tmp/legacy/usr/sbin:/usr/obj/usr/src/tmp/legacy/usr/bin:/usr/obj/usr/src/tmp/leg
acy/bin:/usr/obj/usr/src/tmp/usr/sbin:/usr/obj/usr/src/tmp/usr/bin:/sbin:/bin:/u
sr/sbin:/usr/bin:/usr/local/bin:/usr/local/sbin  SRC_BASE=/usr/src  OSVERSION=12
00000  WRKDIRPREFIX=/usr/obj/usr/src/sys/ make -B clean all
===>  Cleaning for virtualbox-ose-kmod-5.0.26_1
===>  License GPLv2 accepted by the user
===>  Found saved configuration for virtualbox-ose-kmod-4.3.34
===>   virtualbox-ose-kmod-5.0.26_1 depends on file: /usr/local/sbin/pkg - found
===> Fetching all distfiles required by virtualbox-ose-kmod-5.0.26_1 for buildin
g
===>  Extracting for virtualbox-ose-kmod-5.0.26_1
=> SHA256 Checksum OK for VirtualBox-5.0.26.tar.bz2.
===>  Patching for virtualbox-ose-kmod-5.0.26_1
===>  Applying FreeBSD patches for virtualbox-ose-kmod-5.0.26_1
===>   virtualbox-ose-kmod-5.0.26_1 depends on executable: kmk - found
===>  Configuring for virtualbox-ose-kmod-5.0.26_1
Checking for environment: Determined build machine: freebsd.amd64, target machin
e: freebsd.amd64, OK.
Checking for kBuild: found, OK.
Checking for gcc:
  ** cc -target x86_64-unknown-freebsd12.0 --sysroot (variable CC) not found!
Check /usr/obj/usr/src/sys/usr/ports/emulators/virtualbox-ose-kmod/work/VirtualB
ox-5.0.26/configure.log for details
===>  Script "configure" failed unexpectedly.
Please report the problem to vbox_at_FreeBSD.org [maintainer] and attach the
"/usr/obj/usr/src/sys//usr/ports/emulators/virtualbox-ose-kmod/work/VirtualBox-5
.0.26/config.log"


It appears that the problem is due to CC being set to:
	cc -target x86_64-unknown-freebsd12.0 --sysroot
and the Makefile for the port passes this:
	--with-gcc="${CC}"
to configure.  The configure script passes $CC to check_avail, which
does a -z test on it.

I think that CC should just be set to "cc" and the rest should get added
to CFLAGS.  I suspect this got broken by the recent crossbuild changes.