http://*.com/questions/6595208/what-does-this-line-mean-local-export-c-includes

LOCAL_EXPORT_CFLAGS
Define this variable to record a set of C/C++ compiler flags that will
be added to the LOCAL_CFLAGS definition of any other module that uses
this one with LOCAL_STATIC_LIBRARIES or LOCAL_SHARED_LIBRARIES.

LOCAL_EXPORT_CFLAGS定义了一组C/C++编译器flags，当其他模块以LOCAL_STATIC_LIBRARIES/LOCAL_SHARED_LIBRARIES方式引用该模块时，就会将该组值加入到LOCAL_CFLAGS，从而传递给编译器。

LOCAL_EXPORT_C_INCLUDES和LOCALC_INCLUDES：

编译某模块时，如果它依赖别的模块，那么别的模块的LOCAL_EXPORT×的值，会自动加入到本模块。（但是反过来不会有作用）

For example, consider the module 'foo' with the following definition:

   include $(CLEAR_VARS)

   LOCAL_MODULE := foo

   LOCAL_SRC_FILES := foo/foo.c

   LOCAL_EXPORT_CFLAGS :=-DFOO=

   include $(BUILD_STATIC_LIBRARY)

And another module, named 'bar' that depends on it as:

   include $(CLEAR_VARS)

   LOCAL_MODULE := bar

   LOCAL_SRC_FILES := bar.c

   LOCAL_CFLAGS :=-DBAR=

   LOCAL_STATIC_LIBRARIES := foo

   include $(BUILD_SHARED_LIBRARY)

Then, the flags '-DFOO=1 -DBAR=2' will be passed to the compiler when building bar.c

// 这样，编译bar.c文件时，flags -DFOO=1 -DBAR=2将会被传递到编译器。

Exported flags are prepended to your module's LOCAL_CFLAGS so you can
easily override them. They are also transitive: if 'zoo' depends on
'bar' which depends on 'foo', then 'zoo' will also inherit all flags
exported by 'foo'.

// 被导出的flags是继承的，如果zoo依赖bar，bar依赖foo，那么zoo也会继承foo模块中导出的flags。

Finally, exported flags are not used when building the module that
exports them. In the above example, -DFOO=1 would not be passed to the
compiler when building foo/foo.c.

// 被导出的flags对于导出他们的模块是无用的，如上面的例子中，编译foo模块时-DFOO=1不会被传递到编译器。

LOCAL_EXPORT_CPPFLAGS
Same as LOCAL_EXPORT_CFLAGS, but for C++ flags only.

LOCAL_EXPORT_C_INCLUDES
Same as LOCAL_EXPORT_CFLAGS, but for C include paths.
This can be useful if 'bar.c' wants to include headers that are provided by module 'foo'.