Use CMake visibility flags

[remia]

Following recent discussions around symbol visibility, we noticed that OpenColorIO only seem to use -fvisibility-inlines-hidden but not -fvisibility=hidden on the core library. This PR add it and replace the hard coded compiler flags with CMake utility variables to enable that behaviour by default overall.

Here is some tests I did on macOS to illustrate the impact:

cmake -DCMAKE_INSTALL_PREFIX=myinstall -DOCIO_BUILD_APPS=ON -DOCIO_INSTALL_EXT_PACKAGES=ALL .. && cmake --build . -j 12 && make install

// A few moments later..
nm myinstall/lib/libOpenColorIO.2.1.0.dylib | grep " T " | grep -i _XML_ | wc -l
nm myinstall/lib/libOpenColorIO.2.1.0.dylib | grep " T " | wc -l
ls -lh myinstall/lib/libOpenColorIO.2.1.0.dylib | awk '{print $5}'

MASTER BRANCH
66
3318
6.6M

THIS PR
66
907
6.3M

Notice that Expat symbols are still leaked, I don't know yet how to fix that on macOS.

[lgritz]

This is a good improvement that (along with the related PR I made) aligns OCIO with best practices for symbol hiding. Please do also backport this to RB-2.0! Many of us would be relieved to get a 2.0.2 that does all the symbol hiding properly.

[lgritz]

I don't know the right solution for macos, but I would certainly like to find out. What I tried in the other PR works for Linux, but that linker option just didn't exist on Mac.

[hodoulp]

@remia I'm waiting last changes about visibility default values (if that's a valid comment / approach) before approving the pull request. Let me know.