add warnings for header only libraries in izenelib
add warnings for header only libraries in izenelib

Some izenelib depends on other libraries, library end user must also
compile his/her code with the dependent libraries as well.

izenecloud/izenelib,izenecloud/izenelib,pombredanne/izenelib,pombredanne/izenelib,pombredanne/izenelib,izenecloud/izenelib,izenecloud/izenelib,pombredanne/izenelib