The changes in src/exception.cpp and cmake/Modules/HandleLibCXXABI.cmake fix a
bug when building libc++ with GCC. Because GCC does not support __has_include
we need to explicitly tell it that we are building against libc++abi via the
preprocessor definition `LIBCXX_BUILDING_LIBCXXABI`.
The changes in include/ratio are to work around CWG defect
1712 (constexpr variable template declarations). GCC 4.8 and before has not
adopted the resolution to this defect.
The changes in include/exception work around an issue where is_final is used
without it being defined in type_traits.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@237767 91177308-0d34-0410-b5e6-96231b3b80d8
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