Fixing libavcodec.a linking error during build

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I initially built opencv in my ~/opt/ directory with no issues, but then experienced errors using cv2.VideoCapture() via the python bindings. This SO answer led me to rebuild ffmpeg, after which I tried to rebuild opencv in my root /opt/ directory.

I got the following error, and I can't seem to figure out what is causing this. Any suggestions? Thanks for the help!

 [ 18%] Building CXX object modules/videoio/CMakeFiles/opencv_videoio.dir/src/cap_ffmpeg.cpp.o
In file included from /opt/opencv/modules/videoio/src/cap_ffmpeg.cpp:45:0:
/opt/opencv/modules/videoio/src/cap_ffmpeg_impl.hpp: In member function ‘bool OutputMediaStream_FFMPEG::open(const char*, int, int, double)’:
/opt/opencv/modules/videoio/src/cap_ffmpeg_impl.hpp:2230:41: warning: ignoring return value of ‘int avformat_write_header(AVFormatContext*, AVDictionary**)’, declared with attribute warn_unused_result [-Wunused-result]
         avformat_write_header(oc_, NULL);
                                         ^
Linking CXX shared library ../../lib/libopencv_videoio.so
/usr/bin/ld: /usr/local/lib/libavcodec.a(avpacket.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
/usr/local/lib/libavcodec.a: error adding symbols: Bad value
collect2: error: ld returned 1 exit status
make[2]: *** [lib/libopencv_videoio.so.3.1.0] Error 1
make[1]: *** [modules/videoio/CMakeFiles/opencv_videoio.dir/all] Error 2
make: *** [all] Error 2