Introduce _LIBCPP_DEPRECATED_ABI_EXTERNAL_ERROR_CATEGORY_CONSTRUCTOR ABI option.

Currently libc++ compiles a special version of error_category()
into the dylib. This definition is no longer needed, and doesn't
work on Windows due to dllimport/dllexport semantics.

For those reasons this patch introduces an option to
disable/enable this definition. By default the definition
is provided in ABI v1 except on windows. This patch
also addresses D28210.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290840 91177308-0d34-0410-b5e6-96231b3b80d8

src/system_error.cpp

diff --git a/src/system_error.cpp b/src/system_error.cpp
index 3a22554..4ecafea 100644
--- a/src/system_error.cpp
+++ b/src/system_error.cpp
@@ -29,9 +29,11 @@
 
 // class error_category
 
+#if defined(_LIBCPP_DEPRECATED_ABI_EXTERNAL_ERROR_CATEGORY_CONSTRUCTOR)
 error_category::error_category() _NOEXCEPT
 {
 }
+#endif
 
 error_category::~error_category() _NOEXCEPT
 {