fix ltdl detection failure on CentOS6 fixes #3021

libtoolize on the subshell cannot produce exit errors,
so we check as queit dry-run first and put an error to
the user.

tested on centos 6.3 x64, as well as debian wheezy with
temporarly removed /usr/share/libtool/libltdl/COPYING.LIB
Added missing files.