Fix get_dirent_type() for symlinks on MinGW/MSYS.

On Windows with MSVC, get_dirent_type() was recently made to return
DT_LNK for junction points by commit 9d3444dc, which fixed some
defective dirent.c code.

On Windows with Cygwin, get_dirent_type() already worked for symlinks,
as it does on POSIX systems, because Cygwin has its own fake symlinks
that behave like POSIX (on closer inspection, Cygwin's dirent has the
BSD d_type extension but it's probably always DT_UNKNOWN, so we fall
back to lstat(), which understands Cygwin symlinks with S_ISLNK()).

On Windows with MinGW/MSYS, we need extra code, because the MinGW
runtime has its own readdir() without d_type, and the lstat()-based
fallback has no knowledge of our convention for treating junctions as
symlinks.

Back-patch to 14, where get_dirent_type() landed.

Reported-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/b9ddf605-6b36-f90d-7c30-7b3e95c46276%40dunslane.net

src/common/file_utils.c

@@ -465,5 +465,21 @@ get_dirent_type(const char *path,
 #endif
 	}
 
+#if defined(WIN32) && !defined(_MSC_VER)
+
+	/*
+	 * If we're on native Windows (not Cygwin, which has its own POSIX
+	 * symlinks), but not using the MSVC compiler, then we're using a
+	 * readdir() emulation provided by the MinGW runtime that has no d_type.
+	 * Since the lstat() fallback code reports junction points as directories,
+	 * we need an extra system call to check if we should report them as
+	 * symlinks instead, following our convention.
+	 */
+	if (result == PGFILETYPE_DIR &&
+		!look_through_symlinks &&
+		pgwin32_is_junction(path))
+		result = PGFILETYPE_LNK;
+#endif
+
 	return result;
 }