libgflags.a(gflags.cc.o): relocation R_AARCH64_ADR_PREL_PG_HI21

这个问题的解决废了好大功夫，最后google很多论坛才找到方法，特此记录，给大家节省时间

完整版本报错：

/usr/bin/ld: /usr/local/lib/libgflags.a(gflags.cc.o): relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZN22gflags_mutex_namespace5MutexD1Ev' which may bind externally can not be used when making a shared object; recompile with -fPIC
/usr/local/lib/libgflags.a(gflags.cc.o): in function `google::(anonymous namespace)::FlagRegistry::GlobalRegistry()':
gflags.cc:(.text+0x1908): dangerous relocation: unsupported relocation
/usr/bin/ld: /usr/local/lib/libgflags.a(gflags.cc.o): relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZNSt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS5_EED1Ev' which may bind externally can not be used when making a shared object; recompile with -fPIC
/usr/local/lib/libgflags.a(gflags.cc.o): in function `_GLOBAL__sub_I__ZN3fLS14FLAGS_flagfileB5cxx11E':
gflags.cc:(.text.startup+0x2c0): dangerous relocation: unsupported relocation
collect2: error: ld returned 1 exit status
make[2]: *** [modules/sfm/CMakeFiles/opencv_sfm.dir/build.make:242: lib/libopencv_sfm.so.4.5.4] Error 1
make[1]: *** [CMakeFiles/Makefile2:8569: modules/sfm/CMakeFiles/opencv_sfm.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

静态库的链接出问题了，我们重新编译一下并且编译成动态库就好了。
解决方法：重新编译ffmpeg

cd .. && git clone https://github.com/ffmpeg/ffmpeg && cd ffmpeg
./configure --enable-pic -DBUILD_SHARED_LIBS=ON && make && make install

对于在Orin上面的库报错：

undefined reference to `v4l2_open'
undefined reference to `v4l2_mmap'
undefined reference to `v4l2_munmap'
undefined reference to `v4l2_close'

需要在CMakeLists.txt里面手动target_link_libraries链接 v4l1 v4l2（x86平台不需要，我也没搞懂为啥）