Allow the CMake option 'LLVM_ABI_BREAKING_CHECKS' to be undefined.

When building libc++abi in a standalone configuration the CMake option
'LLVM_ABI_BREAKING_CHECKS` will not be defined.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237204 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed