--sysroot 的作用

如果在编译时指定了-sysroot就是为编译时指定了逻辑目录。编译过程中需要引用的库，头文件，如果要到/usr/include目录下去找的情况下，则会在前面加上逻辑目录。

如此处我们指定 -sysroot=/home/shell.albert/tools/toolschain_arm/4.4.3/arm-none-linux-gnueabi/sys-root

则如果在编译过程中需要找stdio.h，则会用/usr/include/目录下去找，因为我们指定了系统目录，则会到下面的路径去找。

LD时候的-rpath,-rpath-link 的意思:

-rpath=dir

Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All -rpath arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The -rpath option is also used when locating shared objects which are needed by shared objects explicitly included in the link; see the description of the -rpath-link option. If -rpath is not used when linking an ELF executable, the contents of the environment variable LD_RUN_PATH will be used if it is defined.

-rpath-link=dir

When using ELF or SunOS, one shared library may require another. This happens when an ld -shared link includes a shared library as one of the input files. When the linker encounters such a dependency when doing a non-shared, non-relocatable link, it will automatically try to locate the required shared library and include it in the link, if it is not included explicitly. In such a case, the -rpath-link option specifies the first set of directories to search. The -rpath-link option may specify a sequence of directory names either by specifying a list of names separated by colons, or by appearing multiple times. 
This option should be used with caution as it overrides the search path that may have been hard compiled into a shared library. In such a case it is possible to use unintentionally a different search path than the runtime linker would do.

The difference between -rpath and -rpath-link is that directories specified by -rpath options are included in the executable and used at runtime, whereas the -rpath-link option is only effective at link time. Searching -rpath in this way is only supported by native linkers and cross linkers which have been configured with the --with-sysroot option.

-L: “链接”的时候，去找的目录，也就是所有的 -lFOO 选项里的库，都会先从 -L 指定的目录去找，然后是默认的地方。
-rpath: “运行”的时候，去找的目录。运行的时候，要找 .so 文件，会从这个选项里指定的地方去找。对于交叉编译，只有配合 --sysroot 选项才能起作用。
-rpath_link （或者 -rpath-link）：这个也是用于“链接”的时候的，例如你显示指定的需要 FOO.so，但是 FOO.so 本身是需要 BAR.so 的，后者你并没有指定，而是 FOO.so 引用到它，这个时候，会先从 -rpath-link 给的路径里找。

也就是说，-rpath指定的路径会被记录在生成的可执行程序中，用于运行时。
-rpath-link 则只用于链接时。

rpath-link的格式：

./configure --cross-prefix=/home/zhou/android/android_adt/ndk/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/arm-linux-androideabi- --sysroot=/home/zhou/android/android_adt/ndk/platforms/android-9/arch-arm --disable-shared --disable-symver --disable-everything --enable-gpl --enable-runtime-cpudetect --enable-decoder=h263 --enable-encoder=h263 --enable-encoder=libx264 --enable-parser=h264 --enable-libx264 --enable-decoder=h264 --prefix=/home/hzh/temp/vpu/build/ffmpeg/ --extra-cflags= -DANDROID -D__thumb__ -mthumb -I../build/x264//include --extra-ldflags= -L../build/x264//lib -Wl,-rpath-link,../build/x264//lib --extra-cxxflags=-Wno-multichar -fno-exceptions -fno-rtti