configure: Explicitly check for pkg-config at the top level

If you don't, then the first place the m4 expands is:

    if test "x$INTEL" != "xno"; then
	    PKG_CHECK_MODULES(PCIACCESS, [pciaccess >= 0.10])
    fi

So on non-Intel architectures we never find it in the path, and all
subsequent PKG_CHECK_MODULESes fail. Boo autoconf.

Signed-off-by: Adam Jackson <ajax@redhat.com>