package/opencv3: link with libatomic if needed

Restore the atomic workaround that was wrongly removed when bumping to
version 3.4.9 in commit f6fb2cae0691d25b034b12a16261ee26096db7d1 as it
seems that opencv3 still needs help to detect atomic library

Fixes:
 - http://autobuild.buildroot.org/results/9162b29725f8d9b891eb74fcb8078f211140a841

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/opencv3/opencv3.mk

@@ -13,6 +13,11 @@ OPENCV3_SUPPORTS_IN_SOURCE_BUILD = NO
 
 OPENCV3_CXXFLAGS = $(TARGET_CXXFLAGS)
 
+# Uses __atomic_fetch_add_4
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+OPENCV3_CXXFLAGS += -latomic
+endif
+
 # Fix c++11 build with missing std::exception_ptr
 ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_64735),y)
 OPENCV3_CXXFLAGS += -DCV__EXCEPTION_PTR=0