package/opencv: add openmp support

Note that this situation is not ideal because it only add openmp support
to the Buildroot toolchain, not the external ones; but a couple of
packages already are in this situation.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/opencv/opencv.mk

@@ -250,6 +250,8 @@ else
 OPENCV_CONF_OPTS += -DWITH_JPEG=OFF
 endif
 
+OPENCV_CONF_OPTS += -DWITH_OPENMP=$(if $(BR2_GCC_ENABLE_OPENMP),ON,OFF)
+
 ifeq ($(BR2_PACKAGE_OPENCV_WITH_PNG),y)
 OPENCV_CONF_OPTS += -DWITH_PNG=ON
 OPENCV_DEPENDENCIES += libpng