Set MACOSX_DEPLOYMENT_TARGET to 10.9 for Mojave

avoid error "ld: library not found for -lstdc++"
see opencv/opencv#8588

Author: novogrammer

binding.gyp

@@ -128,7 +128,8 @@
 				"-std=c++11",
 				"-stdlib=libc++"
 			],
-			"GCC_ENABLE_CPP_EXCEPTIONS": "YES"
+			"GCC_ENABLE_CPP_EXCEPTIONS": "YES",
+			"MACOSX_DEPLOYMENT_TARGET": "10.9"
 		},
 
 		"conditions": [