[v3,2/6] Import filename.h from gnulib

diff --git a/include/filename.h b/include/filename.h
new file mode 100644
index 0000000000..4598fb1d63
--- /dev/null
+++ b/include/filename.h
@@ -0,0 +1,110 @@ 
+/* Basic filename support macros.
+   Copyright (C) 2001-2004, 2007-2020 Free Software Foundation, Inc.
+
+   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 3 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, see <https://www.gnu.org/licenses/>.  */
+
+/* From Paul Eggert and Jim Meyering.  */
+
+#ifndef _FILENAME_H
+#define _FILENAME_H
+
+#include <string.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* Filename support.
+   ISSLASH(C)                  tests whether C is a directory separator
+                               character.
+   HAS_DEVICE(Filename)        tests whether Filename contains a device
+                               specification.
+   FILE_SYSTEM_PREFIX_LEN(Filename)  length of the device specification
+                                     at the beginning of Filename,
+                                     index of the part consisting of
+                                     alternating components and slashes.
+   FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE
+                               1 when a non-empty device specification
+                               can be followed by an empty or relative
+                               part,
+                               0 when a non-empty device specification
+                               must be followed by a slash,
+                               0 when device specification don't exist.
+   IS_ABSOLUTE_FILE_NAME(Filename)
+                               tests whether Filename is independent of
+                               any notion of "current directory".
+   IS_RELATIVE_FILE_NAME(Filename)
+                               tests whether Filename may be concatenated
+                               to a directory filename.
+   Note: On native Windows, OS/2, DOS, "c:" is neither an absolute nor a
+   relative file name!
+   IS_FILE_NAME_WITH_DIR(Filename)  tests whether Filename contains a device
+                                    or directory specification.
+ */
+#if defined _WIN32 || defined __CYGWIN__ \
+    || defined __EMX__ || defined __MSDOS__ || defined __DJGPP__
+  /* Native Windows, Cygwin, OS/2, DOS */
+# define ISSLASH(C) ((C) == '/' || (C) == '\\')
+  /* Internal macro: Tests whether a character is a drive letter.  */
+# define _IS_DRIVE_LETTER(C) \
+    (((C) >= 'A' && (C) <= 'Z') || ((C) >= 'a' && (C) <= 'z'))
+  /* Help the compiler optimizing it.  This assumes ASCII.  */
+# undef _IS_DRIVE_LETTER
+# define _IS_DRIVE_LETTER(C) \
+    (((unsigned int) (C) | ('a' - 'A')) - 'a' <= 'z' - 'a')
+# define HAS_DEVICE(Filename) \
+    (_IS_DRIVE_LETTER ((Filename)[0]) && (Filename)[1] == ':')
+# define FILE_SYSTEM_PREFIX_LEN(Filename) (HAS_DEVICE (Filename) ? 2 : 0)
+# ifdef __CYGWIN__
+#  define FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE 0
+# else
+   /* On native Windows, OS/2, DOS, the system has the notion of a
+      "current directory" on each drive.  */
+#  define FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE 1
+# endif
+# if FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE
+#  define IS_ABSOLUTE_FILE_NAME(Filename) \
+     ISSLASH ((Filename)[FILE_SYSTEM_PREFIX_LEN (Filename)])
+# else
+#  define IS_ABSOLUTE_FILE_NAME(Filename) \
+     (ISSLASH ((Filename)[0]) || HAS_DEVICE (Filename))
+# endif
+# define IS_RELATIVE_FILE_NAME(Filename) \
+    (! (ISSLASH ((Filename)[0]) || HAS_DEVICE (Filename)))
+# define IS_FILE_NAME_WITH_DIR(Filename) \
+    (strchr ((Filename), '/') != NULL || strchr ((Filename), '\\') != NULL \
+     || HAS_DEVICE (Filename))
+#else
+  /* Unix */
+# define ISSLASH(C) ((C) == '/')
+# define HAS_DEVICE(Filename) ((void) (Filename), 0)
+# define FILE_SYSTEM_PREFIX_LEN(Filename) ((void) (Filename), 0)
+# define FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE 0
+# define IS_ABSOLUTE_FILE_NAME(Filename) ISSLASH ((Filename)[0])
+# define IS_RELATIVE_FILE_NAME(Filename) (! ISSLASH ((Filename)[0]))
+# define IS_FILE_NAME_WITH_DIR(Filename) (strchr ((Filename), '/') != NULL)
+#endif
+
+/* Deprecated macros.  For backward compatibility with old users of the
+   'filename' module.  */
+#define IS_ABSOLUTE_PATH IS_ABSOLUTE_FILE_NAME
+#define IS_PATH_WITH_DIR IS_FILE_NAME_WITH_DIR
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _FILENAME_H */