cmSystemTools: Add GetDirCase helper function

On Linux, one can set case insensitivity on a per-directory level.

Source/cmSystemTools.cxx

@@ -118,6 +118,12 @@
 #  include <malloc.h> /* for malloc/free on QNX */
 #endif
 
+#ifdef __linux__
+#  include <linux/fs.h>
+
+#  include <sys/ioctl.h>
+#endif
+
 #if !defined(_WIN32) && !defined(__ANDROID__)
 #  include <sys/utsname.h>
 #endif
@@ -3941,6 +3947,38 @@ std::string cmSystemTools::EncodeURL(std::string const& in, bool escapeSlashes)
   return out;
 }
 
+cm::optional<cmSystemTools::DirCase> cmSystemTools::GetDirCase(
+  std::string const& dir)
+{
+  if (!cmSystemTools::FileIsDirectory(dir)) {
+    return cm::nullopt;
+  }
+#if defined(_WIN32) || defined(__APPLE__)
+  return DirCase::Insensitive;
+#elif defined(__linux__)
+  int fd = open(dir.c_str(), O_RDONLY);
+  if (fd == -1) {
+    // cannot open dir but it exists, assume dir is case sensitive.
+    return DirCase::Sensitive;
+  }
+  int attr = 0;
+  int ioctl_res = ioctl(fd, FS_IOC_GETFLAGS, &attr);
+  close(fd);
+
+  if (ioctl_res == -1) {
+    return DirCase::Sensitive;
+  }
+
+  // FS_CASEFOLD_FD from linux/fs.h, in Linux libc-dev 5.4+
+  // For compat with old libc-dev, define it here.
+  const int CMAKE_FS_CASEFOLD_FL = 0x40000000;
+  return (attr & CMAKE_FS_CASEFOLD_FL) != 0 ? DirCase::Insensitive
+                                            : DirCase::Sensitive;
+#else
+  return DirCase::Sensitive;
+#endif
+}
+
 cmsys::Status cmSystemTools::CreateSymlink(std::string const& origName,
                                            std::string const& newName)
 {

Source/cmSystemTools.h

@@ -584,6 +584,15 @@ public:
   static std::string EncodeURL(std::string const& in,
                                bool escapeSlashes = true);
 
+  enum class DirCase
+  {
+    Sensitive,
+    Insensitive,
+  };
+
+  /** Returns nullopt when `dir` is not a valid directory */
+  static cm::optional<DirCase> GetDirCase(std::string const& dir);
+
 #ifdef _WIN32
   struct WindowsFileRetry
   {