[libc++] Add _LIBCPP_DISABLE_EXTERN_TEMPLATE config option
When the libc++ extern template macros were added, the intent was for it
to be possible for consumers of the headers to disable extern templates
(via `-D_LIBCPP_EXTERN_TEMPLATE(...)=`). Unfortunately, support for
specifying function-like macros varies on the command line varies across
compilers (e.g. MSVC doesn't support it at all), and cmake doesn't allow
it for the same reason. Add a non-function macro for this purpose.
The intended use is for libraries which want to use the libc++ headers
without taking a dependency on the libc++ library itself. I can name the
macro something which reflects its intent rather than its behavior (e.g.
`_LIBCPP_HEADER_ONLY`) if desired.
Differential Revision: https://reviews.llvm.org/D31725
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300246 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed
