Check for pkg-config itself so we don't try the m4 macro without pkg-config.
This helps workaround problem noted on bind10-users list last week and this was discussed on jabber last week. Note that if the m4 macro doesn't exist. Then even if pkg-config is installed later, the configure script will still fail. autoconf will need to be ran again. git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@2106 e5f2f494-b856-4b98-b285-d166d9295462