Link error with VP9 when building a shared library

[tokjun]

The make failed to link VP9 when the OpenIGTLink library is built as a shared library.

[ 34%] Linking CXX shared library ../bin/libOpenIGTLink.so
/usr/bin/ld: ../Deps/VP9/libvpx.a(vpx_codec.c.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
../Deps/VP9/libvpx.a: error adding symbols: Bad value
collect2: error: ld returned 1 exit status
Source/CMakeFiles/OpenIGTLink.dir/build.make:1993: recipe for target 'bin/libOpenIGTLink.so.3.1.0' failed
make[2]: *** [bin/libOpenIGTLink.so.3.1.0] Error 1
CMakeFiles/Makefile2:1036: recipe for target 'Source/CMakeFiles/OpenIGTLink.dir/all' failed
make[1]: *** [Source/CMakeFiles/OpenIGTLink.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2

[tokjun]

Adding --enable-pic to VP9's configure script solves the issue, at least on Linux.

[tokjun]

This error does not exist in Mac OS X. The above change did not affect the building result.