Dependencies: Drop-in replacement for std::filesystem (in extern/)

Introduces the ghc::filesystem library as a (leight weight, header
only) drop-in replacement for std::filesystem. The latter can't be
used on the minimum macOS version we support. Once that changes, we can
switch to using std::filesystem directly.

Part of T90379.

* Add ghc::filesystem to extern/ as platform compatible replacement for
  std::filesystem.
* Add Windows specific code to avoid issues with including Windows.h.
* Add blender::bli::filesystem as alias for ghc::filesystem. That should make
  the ghc -> std transition easier later on since we can just change this
  alias. Also makes it easy to switch between the two at compile time.

extern/ghc_filesystem/README.blender

@@ -0,0 +1,5 @@
+Project: ghc::filesystem
+URL: https://github.com/gulrak/filesystem
+License: MIT
+Upstream version: 1d3d5f5
+Local modifications: None

source/blender/blenlib/BLI_filesystem.hh

@@ -0,0 +1,74 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup bli
+ *
+ * Header for a platform compatible replacement of C++17's `std::filesystem`. Access via
+ * `blender::filesystem`.
+ *
+ * The need for this comes from macOS only supporting `std::filesystem` with XCode 11, only
+ * available on macOS 10.15.
+ */
+
+#pragma once
+
+/* ghc::filesystem includes Windows.h, which by default pollutes the global namespace and impacts
+ * compile time quite a bit. Set some defines to avoid this. */
+#ifdef WIN32
+#  ifndef NOGDI
+#    define NOGDI
+#    define NOGDI_CLEANUP
+#  endif
+#  ifndef NOMINMAX
+#    define NOMINMAX
+#    define NOMINMAX_CLEANUP
+#  endif
+#  ifndef WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN_CLEANUP
+#  endif
+#  ifndef NOCOMM
+#    define NOCOMM
+#    define NOCOMM_CLEANUP
+#  endif
+#endif /* _WIN32 */
+
+/* Header for ghc::filesystem. */
+#include "filesystem.hpp"
+
+#ifdef WIN32
+#  ifdef NOGDI_CLEANUP
+#    undef NOGDI
+#    undef NOGDI_CLEANUP
+#  endif
+#  ifdef NOMINMAX_CLEANUP
+#    undef NOMINMAX
+#    undef NOMINMAX_CLEANUP
+#  endif
+#  ifdef WIN32_LEAN_AND_MEAN_CLEANUP
+#    undef WIN32_LEAN_AND_MEAN
+#    undef WIN32_LEAN_AND_MEAN_CLEANUP
+#  endif
+#  ifdef NOCOMM_CLEANUP
+#    undef NOCOMM
+#    undef NOCOMM_CLEANUP
+#  endif
+#endif
+
+namespace blender {
+namespace filesystem = ghc::filesystem;
+}

source/blender/blenlib/CMakeLists.txt

@@ -26,6 +26,7 @@ set(INC
   ../../../intern/eigen
   ../../../intern/guardedalloc
   ../../../intern/numaapi/include
+  ../../../extern/ghc_filesystem
   ../../../extern/wcwidth
 )