[libc++] fix macro redef warning when exception is disabled

Summary:
 when setting LIBCXX_ENABLE_EXCEPTIONS=false, _LIBCPP_NO_EXCEPTIONS wil be defined in both commandline and _config

Reviewers: bcraig, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19344

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@266956 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed