c++experimental is always created as a static library, but if the
libcxx headers indicate DLL linkage, linking against c++experimental
fails (with unresolved symbols).
Set LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY to OFF by default in such
configurations. If enabled anyway, mark the tests under std/experimental
Remove the explicit LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF from the
examples for how to build the library.