[package] libtool: Unset CXX. It is not needed to build libtool, but if set

configure preferes CXX over CC to build the global_symbol_pipe test program. So
if there is no c++ compiler or the c++ installation is broken we end up with a
broken libtool script. Fixes #5022

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@16212 3c298f89-4303-0410-b956-a3cf2f4a3e73