Use -Wl,defs when linking.

ELF linkers by default allow shared libraries to contain undefined references
and it is up to the dynamic linker to look for them.

On COFF and MachO, that is not the case.

This creates a situation where a .so might build on an ELF system, but the build
of the corresponding .dylib or .dll will fail.

This patch changes the cmake build to use -Wl,-z,defs when linking and updates
the dependencies so that -DBUILD_SHARED_LIBS=ON build still works.

llvm-svn: 226611