[libcxx] Enable filesystem by default for mingw targets

This feature can be built successfully for windows now. However,
the helper functions for __int128_t aren't available in MSVC
configurations, so don't enable it by default there yet. (See
https://reviews.llvm.org/D91139 for discussion on how to proceed
with things in MSVC environments.)

Differential Revision: https://reviews.llvm.org/D97075

Author: mstorsjo

libcxx/CMakeLists.txt

@@ -90,7 +90,11 @@ option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
-if (WIN32)
+if (WIN32 AND NOT MINGW)
+  # Filesystem is buildable for windows, but it requires __int128 helper
+  # functions, that currently are provided by libgcc or compiler_rt builtins.
+  # These are available in MinGW environments, but not currently in MSVC
+  # environments.
   set(ENABLE_FILESYSTEM_DEFAULT OFF)
 endif()
 option(LIBCXX_ENABLE_FILESYSTEM "Build filesystem as part of the main libc++ library"