contrib: define CMAKE_SYSTEM_NAME when building for Android

Otherwise CMake doesn't understand properly it's cross compiling
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/main.mak

@@ -482,6 +482,7 @@ ifdef HAVE_ANDROID
 # Set it to "" right away to short-circuit this behaviour
 	echo "set(CMAKE_CXX_SYSROOT_FLAG \"\")" >> $@
 	echo "set(CMAKE_C_SYSROOT_FLAG \"\")" >> $@
+	echo "set(CMAKE_SYSTEM_NAME Android)" >> $@
 endif
 endif
 	echo "set(CMAKE_C_COMPILER $(CC))" >> $@