do not define android macro for jetson, fix #2939

toolchains/jetson.toolchain.cmake

-# set cross-compiled system type, it's better not use the type which cmake cannot recognized.
-SET ( CMAKE_SYSTEM_NAME Linux )
-SET ( CMAKE_SYSTEM_PROCESSOR aarch64 )
-# for the reason of aarch64-linux-gnu-gcc DONOT need to be installed, make sure aarch64-linux-gnu-gcc and aarch64-linux-gnu-g++ can be found in $PATH: 
-SET ( CMAKE_C_COMPILER "aarch64-linux-gnu-gcc" )
-SET ( CMAKE_CXX_COMPILER "aarch64-linux-gnu-g++" )
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_SYSTEM_PROCESSOR aarch64)
+
+set(CMAKE_C_COMPILER "aarch64-linux-gnu-gcc")
+set(CMAKE_CXX_COMPILER "aarch64-linux-gnu-g++")
 
 # set ${CMAKE_C_FLAGS} and ${CMAKE_CXX_FLAGS}flag for cross-compiled process
 # -march=armv8-a could work on Jetson, but will compile without some extra cpu features
-SET ( CMAKE_CXX_FLAGS "-std=c++11 -march=native -fopenmp ${CMAKE_CXX_FLAGS}" )
+set(CMAKE_C_FLAGS "-march=native ${CMAKE_C_FLAGS}")
+set(CMAKE_CXX_FLAGS "-march=native ${CMAKE_CXX_FLAGS}")
 
-# other settings
-# Jetson CPU supports asimd
-add_definitions ( -D__ARM_NEON)
-# Jetson does NOT run ANDROID
-# but `__ANDROID__` marco is tested before `__aarch64__`
-# and currently no negative effect is caused by this marco
-add_definitions( -D__ANDROID__)
-SET ( ANDROID true)
+# cache flags
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}" CACHE STRING "c flags")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" CACHE STRING "c++ flags")