It's on by default, but can be disabled with the below cmake argument.
Index: Makefile
===================================================================
RCS file: /cvs/ports/graphics/opencv/Makefile,v
retrieving revision 1.37
diff -u -p -u -r1.37 Makefile
--- Makefile 15 Apr 2016 08:53:49 -0000 1.37
+++ Makefile 20 Apr 2016 04:59:34 -0000
@@ -13,6 +13,7 @@ DISTNAME = opencv-$V
PKGNAME-main = ${PKGNAME}
PKGNAME-docs = opencv-docs-$V
PKGNAME-java = opencv-java-$V
+REVISION = 0
REVISION-docs = 0
HOMEPAGE = http://www.opencv.org/
@@ -122,6 +123,7 @@ CONFIGURE_ARGS = -DBUILD_opencv_nonfree:
-DWITH_PNG:Bool=On \
-DWITH_TIFF:Bool=On \
-DWITH_V4L:Bool=Off \
+ -DENABLE_SSE3:Bool=Off \
-DPYTHON_EXECUTABLE:FILEPATH=${MODPY_BIN}
.if ${BUILD_PACKAGES:M-java}
