--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。
-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