btrfs-progs: build: fix undefined macro AC_DEFINE

If I run autoreconf -fi in btrfs-progs repo in VirtualBox, it fails with
possibly undefined macro AC_DEFINE. To fix this, I added m4 as the macro
directory in configure.ac as follows. Adding that line fixes the error
when running autoreconf.

  Versions:
  autoreconf 2.69
  automake 1.15.1
  btrfs-progs v6.3-fba31d63
  openSUSE 15.5
  VirtualBox 6.1.40

  configure.ac:7: error: possibly undefined macro: AC_DEFINE
	If this token and others are legitimate, please use m4_pattern_allow.
	See the Autoconf documentation.
  configure.ac:63: error: possibly undefined macro: AC_MSG_ERROR
  configure.ac:276: error: possibly undefined macro: AC_MSG_WARN
  autoreconf: /usr/bin/autoconf failed with exit status: 1

Issue: kdave#632
Signed-off-by: David Sterba <dsterba@suse.com>

Author: Quipyowert2

configure.ac

@@ -30,6 +30,7 @@ AC_SUBST([CFLAGS])
 AC_PREREQ([2.60])
 
 AC_CONFIG_AUX_DIR([config])
+AC_CONFIG_MACRO_DIR([m4])
 dnl AC_USE_SYSTEM_EXTENSIONS must be called before any macros that run
 dnl the compiler (like AC_PROG_LIBTOOL) to avoid autoconf errors.
 AC_USE_SYSTEM_EXTENSIONS