Add warning when using deprecated compiler versions. Remark out a number of asterisk.m4 check which are currently not used anyway (somewhat faster configure). Add --enable-experimental-xml (future functionality).

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4954 43c1dc8c-776e-4df2-8ef1-6e829299be21