build: ensure that MKINSTALLDIRS is AC_SUBST-defined

build: ensure that MKINSTALLDIRS is AC_SUBST-defined

since we're using gettext-0.14.1, which uses that now-obsolete
automake symbol.  Otherwise, make distcheck would fails like this:

    make[2]: Entering directory `/t/libvirt-0.7.6/_build/po'
    /bin/sh @MKINSTALLDIRS@ /t/libvirt-0.7.6/_inst/share
    /bin/sh: @MKINSTALLDIRS@: No such file or directory
    make[2]: *** [install-data-yes] Error 127

* configure.ac (MKINSTALLDIRS): Define.
For reference, we're currently hamstrung by our desire
to support RHEL5, which still uses gettext-0.14:
http://bugzilla.redhat.com/523713