Mac OS X linker does not support --version-script

[willglynn]

Mac OS X's linker is derived from LLVM, not GNU binutils, and it does not support --version-script.

This commit disables that option for APPLE. There are some possible problems with this:

• It might be too broad, since I understand CMake sets APPLE for all versions of Mac OS. I have no idea what toolchains exist on Mac OS 9 or earlier, so I have no way of knowing if this is correct.
• Without --version-script, there's nothing to restrict which symbols are visible in the resulting libraries.

I don't know if either of these are important. That said, I do know that Mac OS X is broken with both the current release and the current master:

Linking C shared library libz.dylib
ld: unknown option: --version-script
collect2: ld returned 1 exit status
make[2]: *** [libz.1.2.7.dylib] Error 1
make[1]: *** [CMakeFiles/zlib.dir/all] Error 2

It builds with this change.

[madler]

Incorporated.

[breser]

For what it's worth Apple's linker has -exported_symbols_list which can be used for similar purposes, however you can't do it by versions and anything that isn't listed is not exported. There is also a -unexported_symbols_list.

OS X man page for ld is here:
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/ld.1.html