automake: Enable subdir-objects option

When running autoreconf, automake currently reports the message below, followed
by an additional warning for every .c file in a subdir. It is just a warning,
but on Ubuntu this is fatal because warnings are treated as errors.
 See: https://bugs.launchpad.net/ubuntu/+source/partclone/+bug/1283294

Warnings from autoreconf:
  src/Makefile.am:6: warning: source file 'btrfs/crc32c.c' is in a subdirectory,
  src/Makefile.am:6: but option 'subdir-objects' is disabled
  automake: warning: possible forward-incompatibility.
  automake: At least a source file is in a subdirectory, but the 'subdir-objects'
  automake: automake option hasn't been enabled.  For now, the corresponding output
  automake: object file(s) will be placed in the top-level directory.  However,
  automake: this behaviour will change in future Automake versions: they will
  automake: unconditionally cause object files to be placed in the same subdirectory
  automake: of the corresponding sources.
  automake: You are advised to start using 'subdir-objects' option throughout your
  automake: project, to avoid future incompatibilities.
automake: use AC_PROG_MKDIR_P instead of AM_PROG_MKDIR_P

autoreconf emits the warning below. It can be resolved by following the
advice provided by the warning. Though this is just a warning, it is fatal
on Ubuntu because warnings are treated as errors.
 See: https://bugs.launchpad.net/ubuntu/+source/partclone/+bug/1283294

Warnings from autoreconf:
  configure.ac:4: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
  configure.ac:4: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
  configure.ac:4: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.