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diff --git a/m4/ax_extend_srcdir.m4 b/m4/ax_extend_srcdir.m4
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+# ===========================================================================
+#     http://www.gnu.org/software/autoconf-archive/ax_extend_srcdir.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_EXTEND_SRCDIR
+#
+# DESCRIPTION
+#
+#   The AX_EXTEND_SRCDIR macro extends $srcdir by one path component.
+#
+#   As an example, when working in /home/michael/i3-4.12/build and calling
+#   ../configure, your $srcdir is "..". After calling AX_EXTEND_SRCDIR,
+#   $srcdir will be set to "../../i3-4.12".
+#
+#   The result of extending $srcdir is that filenames (e.g. in the output of
+#   the "backtrace" gdb command) will include one more path component of the
+#   absolute source path. The additional path component makes it easy for
+#   users to recognize which files belong to the PACKAGE, and -- provided a
+#   dist tarball was unpacked -- which version of PACKAGE was used.
+#
+#   As an example, in "backtrace", you will see:
+#
+#     #0  main (argc=1, argv=0x7fffffff1fc8) at ../../i3-4.12/src/main.c:187
+#
+#   instead of:
+#
+#     #0  main (argc=1, argv=0x7fffffff1fc8) at ../src/main.c:187
+#
+#   In case your code uses the __FILE__ preprocessor directive to refer to
+#   the filename of the current source file (e.g. in debug messages), using
+#   the extended path might be undesirable. For this purpose,
+#   AX_EXTEND_SRCDIR defines the output variable AX_EXTEND_SRCDIR_CPPFLAGS,
+#   which can be added to AM_CPPFLAGS in Makefile.am in order to define the
+#   preprocessor directive STRIPPED__FILE__. As an example, when compiling
+#   the file "../../i3-4.12/src/main.c", STRIPPED__FILE__ evaluates to
+#   "main.c".
+#
+#   There are some caveats: When $srcdir is "." (i.e. when ./configure was
+#   called instead of ../configure in a separate build directory),
+#   AX_EXTEND_SRCDIR will still extend $srcdir, but the intended effect will
+#   not be achieved because of the way automake specifies file paths:
+#   automake defines COMPILE to use "`test -f '$source' || echo
+#   '\$(srcdir)/'`$source" in order to prefer files in the current directory
+#   over specifying $srcdir explicitly.
+#
+#   The AX_EXTEND_SRCDIR author is not aware of any way to influence this
+#   automake behavior. Patches very welcome.
+#
+#   To work around this issue, you can use AX_ENABLE_BUILDDIR i.e. by adding
+#   the following code to configure.ac:
+#
+#     AX_ENABLE_BUILDDIR
+#     dnl ...
+#     AX_EXTEND_SRCDIR
+#
+#   Then also add this bit to Makefile.am (if you wish to use
+#   STRIPPED__FILE__ in your code):
+#
+#     AM_CPPFLAGS = @AX_EXTEND_SRCDIR_CPPFLAGS@
+#
+# LICENSE
+#
+#   Copyright (c) 2016 Michael Stapelberg <michael@i3wm.org>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 3
+
+AC_DEFUN([AX_EXTEND_SRCDIR],
+[dnl
+AS_CASE([$srcdir],
+  [.|.*],
+  [
+    # pwd -P is specified in IEEE 1003.1 from 2004
+    as_dir=`cd "$srcdir" && pwd -P`
+    as_base=`AS_BASENAME([$as_dir])`
+    srcdir=${srcdir}/../${as_base}
+
+    AC_SUBST([AX_EXTEND_SRCDIR_CPPFLAGS], ["-DSTRIPPED__FILE__=AS_ESCAPE([\"$$(basename $<)\"])"])
+  ])
+])dnl AX_EXTEND_SRCDIR