Make visibility tests work as intended on Mac OS X

If the tests detect that visibiliy works, then use it also in
gbuild. In the old build system, -fvisibility=hidden was already
being used on Mac OS X if HAVE_GCC_VISIBILITY_FEATURE had been
detected.

In configure.in, let's not hardcode the -shared, -fpic and -Wl,-z,defs
options or the .so suffix used in visibility-related tests. Factor
them out and use platform-specific options. Done just for Mac OS X so
far. Using the Linux options for Mac OS X caused visibility tests to
silently and misleadingly fail.

Yes, it is silly to now define some platform-specific options in three
places: configure.in, solenv/inc/* for the old build system, and
solenv/gbuild/platform/* for gbuild.

At least with my Xcode3 installation, I need to pass an -isysroot flag
pointing to the SDK when running $CC -E, otherwise headers weren't
found. This was then misinterpreted as the visibility tests failing.

Pass -DHAVE_GCC_VISIBILITY_FEATURE to the compiler if configure
detected visibility working. In that case also pass
-fvisibility=hidden. HAVE_GCC_VISIBILITY_FEATURE being defined is
supposed to mean that the -fvisibility=hidden option is used, I think.

Pass also -fvisibility-inlines-hidden if that was detected to work.

Change-Id: I I I58d566fcb07584246e91f45e683ce9b31208edba

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