[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

llvm-svn: 266956