opencv: no symbols warnings while building opencv

While investigating #4576 , I found that there were lots of missing symobls warning during building opencv by apothecary update ofxOpenCv. A part of warnings is:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: i386 file: ../../lib/libopencv_highgui.a(grfmt_imageio.cpp.o) h
as no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: i386 file: ../../lib/libopencv_highgui.a(grfmt_jpeg.cpp.o) has
no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: i386 file: ../../lib/libopencv_highgui.a(grfmt_jpeg2000.cpp.o)
has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: i386 file: ../../lib/libopencv_highgui.a(grfmt_png.cpp.o) has n
o symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: x86_64 file: ../../lib/libopencv_highgui.a(grfmt_imageio.cpp.o)
 has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: x86_64 file: ../../lib/libopencv_highgui.a(grfmt_jpeg.cpp.o) ha
s no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: x86_64 file: ../../lib/libopencv_highgui.a(grfmt_jpeg2000.cpp.o
) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: for architecture: x86_64 file: ../../lib/libopencv_highgui.a(grfmt_png.cpp.o) has
 no symbols

The full log can be found at my gist (note that the log was generated by a slightly modified build script; added -DWITH_EIGEN=OFF option to build opencv in my local).

I'm not sure if this causes some problem or not, but removing warnings is better than keeping them.

FYI: needs https://github.com/openframeworks/openFrameworks/pull/4577 to save stderr to the log file.

Start by reproducing apothecary update ofxOpenCv and review the linked gist for the complete warning output. Inspect the build script, including the -DWITH_EIGEN=OFF variation, and consider the referenced pull request about saving stderr to the log. Done means the cause of the missing-symbol warnings is understood and the OpenCV build completes without unnecessary warnings.