revamped configure system ... lots more stuff is in acinclude now still a

bit of a mess, but better than before ... if configure does not find the
dependent libraries and header it will not try to get help from pkg-config.

If you know where your libraries are it is best to just use CPPFLAGS and
LDFLAGS to specify their location.

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@306 a5681a0c-68f1-0310-ab6d-d61299d08faa