Avoid splitting Autoconf check message for libparted version (#740004)

First use of PKG_CHECK_EXISTS causes additional checking messages to be
reported which splits the libparted version check message from it's
result, like this:

    checking for libparted >= 1.7.1 (querying pkg-config)... checking for pkg-config... /usr/bin/pkg-config
    checking pkg-config is at least version 0.9.0... yes
    2.3

Call PKG_CHECK_EXISTS early, before first use to avoid this.  Output
now looks like:

    checking for pkg-config... /usr/bin/pkg-config
    checking pkg-config is at least version 0.9.0... yes
    :
    checking for libparted >= 1.7.1 (querying pkg-config)... 2.3

Bug 740004 - use pkg-config to check for version of libparted