configmake: support new --runstatedir option

configmake: support new --runstatedir option

http://lwn.net/Articles/436012/ documents that many distros
are now preferring to use /run rather than /var/run for
storage of pid files and other per-process temporary files
that must not be cleaned out during arbitrary TMPDIR sweeps.
As such, the GNU Coding Standards were recently changed to
recommend a new configure option to make it easy to choose
this directory at configure time, and autoconf 2.70 will
support the new directory by default.  This patch adds support
for propagating results of the new option (for new enough
autotools) or providing a sane default (for older autotools)
into C code.

* m4/configmake.m4 (gl_CONFIGMAKE_PREP): Substitute runstatedir
even if autoconf was too old to provide the command line option.
* modules/configmake (Makefile.am): Propagate it to .h file.

Signed-off-by: Eric Blake <eblake@redhat.com>